TESTING AND TESTING APPARATUS
The practical electrician frequently has to make tests of various kinds
which require the rapid and accurate measurement of voltage, current
and resistance. It is therefore essential that he understand the methods
employed in testing and the operation of the instruments used.
Most tests are made with a galvanometer, and the devices, such as
resistances, switches, etc., which are used in connection with the
galvanometer may be obtained put up in a neat and substantial box together
with the galvanometer, the combination being called a "testing set."
Numerous forms of testing set are illustrated in this chapter.
The construction, use, and operation of the various types of galvanometer
have been explained in chapter twenty-six. Ammeters and voltmeters, which
are simply special forms of galvanometer, and which are largely used are
fully described in the preceding chapter.
Pressure Measurement.--An electromotive force has been defined as that
which causes or tends to cause a current; it is analogous to water
pressure; potential difference corresponds to difference of level. The
_total_ electromotive force of a circuit is independent of resistance or
current, and cannot be limited to mean the fall of pressure between any
two points, as for instance the terminals of a battery.
If the pressure of a battery be two volts when measured on open
circuit by a static voltmeter, there will still be two volts on closed
circuit, but there will now be a loss of pressure through the internal
resistance of the battery and the voltage across the terminals will
be less than the _total_ voltage. The static voltmeter, never closing
the circuit, actually measures the total voltage.
[Illustration: Fig. 540.--Clark cell (Kahle's modification of the Rayleigh
H form), the standard for the International volt. The cell has for its
positive electrode, mercury, and for its negative electrode, amalgamated
zinc. The electrolyte consists of a saturated solution of zinc sulphate
and mercurous sulphate. The pressure is 1.434 volts at 15°C., and between
10°C. and 25°C. the pressure decreases .00115 of a volt for each increase
of 1°C. The containing glass vessel consists of two limbs, closed at
bottom and joined above to a common neck fitted with a ground glass
stopper. The diameter of the limbs should be at least 2 cms., and their
length at least 3 cms. The neck should be not less than 1.5 cms. in
diameter. At the bottom of each limb a platinum wire of about .4 mm. in
diameter is sealed through the glass. To set up the cell, place mercury in
one limb, and in the other hot liquid amalgam, containing 90 parts mercury
and 10 parts zinc. The platinum wires at the bottom must be completely
covered by the mercury and the amalgam, respectively. On the mercury,
place a layer 1 cm. thick of the zinc and mercurous sulphate paste.
Both this paste and the zinc amalgam must be covered with a layer of the
neutral zinc sulphate crystals 1 cm. thick. The whole vessel must then be
filled with the saturated zinc sulphate solution, and the stopper inserted
so that it shall just touch it, leaving, however, a small bubble to guard
against breakage when the temperature rises. Before finally inserting the
glass stopper a strong alcoholic solution of shellac is applied to the
upper edge, after which the stopper is pressed firmly in place.]
Ques. What error is introduced in measuring the pressure of a battery with
an ordinary voltmeter?
Ans. Since the measurement is made on _closed circuit_ the reading does
not give the total pressure of the battery.
The error is very slight because the resistance of the voltmeter is
very high and the current so small that the loss of pressure in the
battery can be neglected.
[Illustration: Fig. 541.--Weston Cadmium Cell. It is made in two forms;
one known as the Weston normal cell, in which the solution of cadmium
sulphate is saturated at all temperatures at which the cell may be used.
The other, known as the Weston standard cell, in which the cadmium
sulphate solution is unsaturated at all temperatures above 4°C. The
Weston normal cell, or saturated form is slightly affected by changes
in temperature, but, on account of the fact that it can be accurately
reproduced, it was adopted by the London Conference in 1908, as a
convenient voltage standard. The value of its voltage suggested by the
committee of the London Conference on Electrical Units and Standards,
and adopted by the Bureau of Standards at Washington, Jan. 1st, 1911, is
1.0183 International volts at 20°C. At any other temperature its voltage
is:
E_{t} = E_{20} - 0.0000406(t-20) - 0.00000095(t-20)^{2} + 0.00000001(t-20)^{3}
The Weston standard cell, or unsaturated form is practically unaffected by
changes in temperature and is the form most commonly used for laboratory
work and general testing. The average pressure of this form is 1.0187 Int.
volts.]
Ques. Define the International volt.
Ans. _It is the electromotive force that, steadily applied to a conductor
whose resistance is one International ohm, will produce a current of
one International ampere, and which is represented sufficiently well for
practical use by 1,000/1,434 of the voltage between the poles of the Clark
cell at a temperature of 15° C., when prepared as in fig. 540._
The relation between the units volt, ampere and ohm, are shown
graphically in figs. 542 and 543.
[Illustration: Figs. 542 and 543.--Diagrams showing hydraulic analogy
illustrating the difference between amperes and coulombs. The _rate_
of current flow of one ampere in fig. 543 may be compared to the rate
of discharge of a pump as in fig. 542. Assuming the pump to be of such
size that it discharges a gallon per stroke and is making 60 strokes
per minute, the quantity of water discharged per hour (coulombs in fig.
543) is 1 × 60 × 60 = 3,600 gallons. Following the analogy further (in
fig. 543), the pressure of one volt is required to force the electricity
through the resistance of one ohm between the terminals A and B. In fig.
542, the boiler must furnish steam pressure on the pump piston to overcome
the friction (resistance) offered by the pipe and raise the water from the
lower level A' to the higher level B'. The difference of pressure between
A and B in the electric circuit corresponds to the difference of pressure
between A' and B'. The cell furnishes the energy to move the current by
maintaining a difference of pressure at its terminals C and D; similarly,
the boiler furnishes energy to raise the water by maintaining a difference
of pressure between the steam pipe C and exhaust pipe D'.]
[Illustration: Fig. 543. If the current strength in fig. 543 be one
ampere, the quantity of electricity passing any point in the circuit per
hour is 1 × 60 × 60 = 3,600 coulombs.]
Current Measurement.--It is necessary to adopt some arbitrary standard in
order to compare currents of different strengths. The term _strength of
a current_, or current strength means the _rate of flow_ past any point
in the circuit in a given unit of time. The unit of current, called the
_ampere_, is defined as _the unvarying current which, when passed through
a solution of nitrate of silver in water (15 per cent. by weight of the
nitrate) deposits silver at the rate of .001118 gramme per second_.
[Illustration: Fig. 544.--Queen weight voltameter for determining the
strength of current by the weight of metal deposited in a given time.
The two outside plates form the anode and are joined together and to one
binding post, while the cathode is placed between them and connected to
the other binding post. The cathode thus receives a deposit on both sides.
An adjustable arm serves to lower the plates into the electrolyte. To
calculate the strength of an unknown current which has passed through a
weight voltameter, _divide the gain in weight by the number of seconds
the current flows through the instrument and by the weight deposited by
one ampere in one second_. That is, current strength in amperes = gain in
weight ÷ (time in seconds × .0003286).]
Ques. How much copper or zinc will one ampere deposit in one second?
Ans. .0003286 gramme of copper in a copper voltameter, or .0003386 gramme
of zinc in a zinc voltameter.
Ques. What is the difference between an ampere and a coulomb?
Ans. An ampere is the unit _rate of flow_ of the current, and a coulomb
is the unit _quantity_ of electricity, that is, the ampere is the rate of
current flow that will deposit .0003286 grammes of copper in one second
and a coulomb is the _quantity_ of electricity that passes a given point
in one second when the current strength is one ampere. In other words a
coulomb is one _ampere second_.
[Illustration: Fig. 545.--Gas voltameter for determining the strength of
current by the volume of gas evolved. To use, connect up as shown in the
illustration. Adjust so that the zero position of the burette is about
one-half inch below the level of the top of the U tube. Pour acidulated
water into the mouth of the burette till the water in the U tube is about
one-half inch from the top. With the electrodes inserted through the
corks, _carefully_ place each one in position by giving a slight twist to
the right as the cork enters. The water level in the U tube and burette
should now be the same, or further adjustment must be made to attain this
result. The level in the burette does not necessarily have to correspond
with the zero graduation, but must not be below it. Unclamp the burette
and hold it nearly horizontal. The liquid will not run out if the corks
be tight, so that this is the _air leakage test_. Attach the connectors
and wires from the current source (which should have a pressure of 2 or
more volts) placing a switch in the circuit. When the switch is closed,
bubbles of gas will rise in the U tube from both electrodes, displacing
the water and forcing it up the burette. Hydrogen will be liberated over
the negative electrode, and oxygen over the positive electrode in the
proportion of twice as much hydrogen as oxygen. To calculate the current
strength, _divide the volume of gas liberated by the time in seconds, and
by the volume of gas liberated (in cubic centimeters) by one ampere in one
second and by .1733_; that is: amperes = volume of gas liberated ÷ (time
in seconds × .1733).]
EXAMPLE.--If an arc lamp require a current of 8 amperes, how much
electricity does it consume per hour?
Since one coulomb = one ampere second, the quantity of electricity
consumed per hour is
8 amperes × ( 60 × 60 ) = 28,800 coulombs.
Voltameter.--A voltameter is an electrolytic cell employed to measure
an electric current by the amount of chemical decomposition the current
causes in passing through the cell. There are two classes of voltameter:
1. Weight voltameters;
2. Gas voltameters.
Ques. What is the difference between these two classes of voltameter?
Ans. In one, the current strength is determined by the weight of metal
deposited or weight of water decomposed, and in the other by the volume
of gas liberated.
Fig. 544 shows a weight voltameter and fig. 545 a gas voltameter.
Ques. How should the plates of a weight voltameter be treated before use?
Ans. They must be thoroughly cleaned and polished with sandpaper, the
sand being afterwards removed by placing them in running water. _The
fingers must not be placed on any part of the plate which is to receive
the deposit._
Ques. What form of voltameter has been selected to measure the
International ampere?
Ans. The silver voltameter arranged as here specified:
The cathode on which the silver is to be deposited shall take the form
of a platinum bowl, not less than 10 cms. in diameter, and from 4 to
5 cms. in depth.
The anode shall be a disc or plate of pure silver some 30 sq. cms.
in area, and 2 or 3 cms. in thickness. This shall be supported
horizontally in the liquid near the top of the solution by a silver
rod riveted through its center.
To prevent the disintegrated silver which is formed on the anode
falling upon the cathode, the anode shall be wrapped around with pure
filter paper, secured at the back by suitable folding.
The liquid shall consist of a neutral solution of pure silver nitrate
containing about 15 parts by weight of the nitrate to 85 parts of
water.
Ques. What is the value of the International ampere as measured with the
silver voltameter?
Ans. The International ampere is represented sufficiently well for
practical use by the unvarying current which, when passed through a silver
voltameter (as described above) deposits _silver at the rate of .001118
gramme per second_.
[Illustration: Fig. 546.--Single contact and short circuiting key. This
key is intended especially for use with D'Arsonval galvanometers in zero
deflection methods. The key is connected in circuit with the galvanometer
so that whenever the key is not depressed, the galvanometer is short
circuited, and its oscillations quickly damped out by the currents induced
in its coil. The back end of the spring is held in a slot in a rubber
block attached to the base.]
Ohm's Law and the Ohm.--The various tests here described depend for their
truth upon the definite relation existing between the electric current,
its pressure, and the resistance which the circuit offers to its flow.
This relation was fully investigated by Ohm in 1827. Using the same
conductor, he proved not only that the current varies with the pressure,
but that it varies in direct proportion.
Ohm's law has already been discussed in a previous chapter and the several
ways of expressing it are repeated here for convenience:
volts
1. Amperes = -------;
ohms
2. Volts = amperes × ohms;
volts
3. Ohms = ---------.
amperes
Various values have been assigned, from time to time, to the ohm or unit
of resistance, the unit in use at the present time being known as the
_International ohm_. This was recommended at the meeting of the British
Association in 1892, was adopted by the International Electrical Congress
held in Chicago in 1893, and was legalized for use in the United States
by act of Congress in 1894. The International ohm in graphically defined
in fig. 548. The previous values given to the ohm which were more or less
generally accepted are as follows:
The Siemens' Ohm.--A resistance due to a column of mercury 100 cm. long
and 1 sq. mm. in cross section at 0° C.
B. A. (British Association) Ohm.--A resistance due to a column of mercury
approximately 104.9 cm. long and 1 sq. mm. in cross section at 0° C.
Legal Ohm.--A resistance due to a column of mercury 106 cm. long and
1 sq. mm. in cross section at 0° C. This unit was adopted by the Paris
conference of 1884.
OHM TABLE[A]
| | Inter-| | | |
| Date |national| Legal|B. A. |Siemens'|
| | Ohm| Ohm|Ohm | Ohm|
-------------------------+------+--------+--------+--------+--------|
International Ohm |1893-4| 1.0000| 1.0028| 1.0136| 1.0630|
Legal Ohm | 1884 | .9972| 1.0000| 1.0107| 1.0600|
B. A. Ohm | 1864 | .9866| .9894| 1.0000| 1.0488|
Siemens' Ohm | | .9407| .9434| .9535| 1.0000|
[A] NOTE.--In the above table to reduce, for instance, British Association
ohms to International ohms, multiply by .9866, or divide by 1.0136; to
reduce legal ohms to International ohms, multiply by .9972, or divide by
1.0028, etc.
[Illustration: Fig. 547.--Double contact key. It is of especial value in
connection with a Wheatstone bridge. When used with the latter it forms
a combination battery and galvanometer key. The battery is wired to the
top leaves of the key and the galvanometer to the lower leaves. Hence,
when operated, the battery circuit will be closed before the galvanometer
circuit, as it is desirable to avoid undue disturbance of the needle.]
[Illustration: Fig. 548.--The international ohm. It is defined as the
resistance of 14.452 grammes of mercury in the form of a column of uniform
cross section 106.3 centimeters in length, at a temperature of 0° C. This
is approximately equivalent to a column 106.3 cm. long, having a uniform
cross section of 1 sq. mm. In the figure the international ohm is defined
graphically. The resistance of the external circuit and the standard one
volt cell is assumed to be zero.]
[Illustration: Fig. 549.--Leeds and Northrup one ohm standard resistance
(Reichsanstalt form); adjusted at 20° C. Low resistance standards may be
properly divided into two classes: 1. those which are designed primarily
as resistance standards, and 2. those designed as current carrying
standards. Those of the first mentioned class are often used to measure
currents up to their capacity. The above standard has both pressure and
current terminals. The binding posts for the former are mounted on high
posts so as to be easily accessible when the standard is immersed in
oil. When used as a resistance standard of precision, it should not be
subjected to a current of more than one ampere, and when used as a current
carrying standard of lesser accuracy, a current of 2 or 3 amperes may be
used.]
Practical Standards of Resistance.--The column of mercury as shown in fig.
548, is the recognized standard for resistance, however, in practice, it
is not convenient to compare resistances with such a piece of apparatus,
and therefore secondary standards are made up and standardized with a
great degree of precision. These secondary standards are made of wire.
The material generally used being manganin or platinoid.
[Illustration: Fig. 550.--Direct deflection method of testing
resistances; a useful and simple method which may be used in numerous
tests. Galvanometer readings are taken through the known, and unknown
resistances, and the current being proportional to the deflections, the
value of the unknown resistance is easily calculated.]
Resistance Measurement.--_Resistance is that which offers opposition to
the flow of electricity._ Ohm's law shows that the strength of the current
falls off in proportion as the resistance in the circuit increases. This
gives a basis for measuring resistance.
There are various methods by which an unknown resistance may be measured,
as by the:
1. Direct deflection method;
2. Method of substitution;
3. Fall of potential method;
4. Differential galvanometer method;
5. Drop method;
6. Voltmeter method;
7. Wheatstone bridge method.
Direct Deflection Method.--This method is based on the fact that the
greater the current through a galvanometer the greater the deflection of
the needle; it is a simple method and is capable of extended application.
The apparatus required consists of battery, galvanometer, known
resistance, and double contact key. The connections are made as in fig.
550. The known resistance is put in circuit with the galvanometer and
after noting the deflection, the key is moved so as to cut out the known
resistance and throw into circuit the unknown resistance. The deflection
of the galvanometer is again noted and compared with the first deflection.
[Illustration: Fig. 551.--Charge and discharge key, adapted to condenser
testing where the condenser is to be alternately charged and discharged.
The insulated handle enables the key to be used without insulating the
body.]
[Illustration: Fig. 552.--Pohl commutator. This is equivalent to a two
pole double throw switch. The depressions in the base are filled with
mercury into which the contacts dip in closing the circuit.]
If the deflections be proportional to the current, the unknown resistance
will be as many times the known resistance as the deflection with
the known resistance is greater than the deflection with the unknown
resistance.
Method of Substitution.--This is the simplest method of measuring
resistance. The resistance to be measured is inserted in series with a
galvanometer and some constant source of current, and the galvanometer
deflection noted. A known adjustable resistance is then substituted for
the unknown and adjusted till the same deflection is again obtained. The
value of the adjustable resistance thus obtained is equal to that of the
resistance being tested.
[Illustration: Fig. 553.--Substitution method of testing resistances.
The connections and apparatus are the same as in fig. 550, except that
a resistance box is used in place of the known resistance. In making the
test, first note deflection with unknown resistance in circuit, then press
key so that the current will pass through the resistance box, and adjust
the resistance in the box so that the deflection of the galvanometer is
about the same as with the unknown. Now switch from one circuit to the
other, changing the resistance in the box until equal deflections are
obtained. When this obtains, the resistance in the box is the same as the
resistance being tested.]
Ques. What kind of adjustable resistance is used in making the above test?
Ans. A resistance box.
Ques. Describe a resistance box.
Ans. It consists of a box containing numerous resistance coils with their
ends connected to terminals and provided with plugs so that they may be
thrown into or out of circuit at will, thus varying the resistance in the
circuit.
[Illustration: Fig. 554.--Ordinary resistance box. It contains sets of
standard resistances consisting of coils of insulated wire having low
conductivity and small temperature coefficient. The ends of the coils are
joined to the section of the bar between the plugs. The insertion of a
plug cuts out a coil. In using, care should be taken to put the plugs in
with a slight twist so that there shall be no resistance introduced by
poor contact.]
Fall of Potential Method.--This is a very simple method of measuring
resistances, and one that is convenient for practical work in electrical
stations because it requires only an ammeter, voltmeter, battery and
switch--apparatus to be found in every station. The connections are made
as shown in fig. 555.
In making the test the ammeter and voltmeter readings are taken at
the same time, and the unknown resistance calculated from Ohm's law.
Accordingly, since:
(1) amperes = volts / ohms
solving for the resistance,
(2) ohms = volts / amperes
[Illustration: Fig. 555.--Fall of potential method of testing resistances;
a convenient method for testing at stations, requiring only the usual
instruments to be found at a station. The resistance of the voltmeter must
be very high, otherwise the test must be made as in fig. 556.]
EXAMPLE.--If in fig. 555 the readings show 6 volts and 2 amperes how many
ohms is the resistance being tested?
Substituting in formula (2)
ohms = 6/2 = 3
Ques. Can this test be made with any kind of voltmeter?
Ans. Its resistance must be very high to avoid error. When a voltmeter
having small resistance is used, it should be connected so as to measure
the fall of pressure across both ammeter and unknown resistance as shown
in fig. 556.
[Illustration: Fig. 556.--Fall of potential method of testing resistances;
diagram showing connections for testing with low resistance voltmeter.
The resistance measured with this connection will be the sum of the
resistances of the coil and the ammeter. The resistance of the ammeter is
usually known and can be subtracted from the sum to obtain the required
resistance.]
Differential Galvanometer Method.--This is what is known as a _nil_ or
zero method, that is, a method of making electrical measurements in which
comparison is made between two quantities by reducing one to equality with
the other, the absence of deflection from zero of the instrument scale
showing that the equality has been obtained.
The test is made with a differential galvanometer, and resistance box
connected as in fig. 557. The current then will divide so that part of
it flows through the resistance being tested and around one set of coils
of the galvanometer while the other part will flow through the resistance
box and the other set of coils as indicated. When the resistance box has
been so adjusted that its resistance is the same as the unknown resistance
the current in the two branches will be equal, and the needle of the
galvanometer will show _no deflection_.
[Illustration: Fig. 557.--Differential galvanometer method of testing
resistances. In making the test, the resistance box is adjusted till the
galvanometer needle shows no deflection. When this condition obtains,
the resistance in circuit in the resistance box is equal to the unknown
resistance, hence, a reading of the box gives the value of the unknown
resistance.]
Ques. What name is given to this method of testing?
Ans. It is called a _zero_ method, distinguishing it from _deflection_
methods.
Ques. For what kind of resistance is the method adapted?
Ans. Since it is a nil or zero method, it is better adapted to the
measurement of non-inductive than of inductive resistances.
Ques. What precaution should be taken with inductive resistances?
Ans. The current must be allowed to flow until it becomes steady to
overcome the influence of self-induction.
Ques. What may be said with respect to the differential galvanometer
method?
Ans. With an accurate instrument it is very reliable.
[Illustration: Fig. 558.--Drop method of testing resistances. The
apparatus is connected as shown and readings taken with voltmeter across
known and unknown resistance. The unknown resistance is then easily
calculated.]
Drop Method.--This is a convenient method, and one which may be used for
measuring either high or low resistances with precision. It is used for
many practical measurements, and requires only a voltmeter, battery, known
resistance and a two way switch.
The instruments are connected as in fig. 558, and in making the test,
the voltmeter is switched into circuit across the known resistance and
then across the unknown resistance, readings being taken in each case.
The value of the unknown resistance, is then easily calculated from the
following proportion:
drop across known resistance known resistance
------------------------------ = ------------------
drop across unknown resistance unknown resistance
from which
unknown known resistance × drop across unknown resistance
resistance = ---------------------------------------------------------
drop across known resistance
[Illustration: Fig. 559.--Leeds and Northrup portable galvanometer
(pointer type A). The sensitiveness of this instrument is such that it may
be substituted in numerous cases for the non-portable reflecting type of
galvanometer; as for instance, in the checking of ammeters and voltmeters
to an accuracy of .2% by the potentiometer method, and on almost all
Wheatstone bridge measurements to commercial accuracies. A current of 2
micro-amperes will cause the pointer to move 1 mm. over the scale, that
is, it has a sensibility of 500,000 ohms. The method of suspending the
moving system is such as to practically eliminate initial friction which
is of value in all zero deflecting methods. The suspensions and moving
system are guarded by springs, which together with the solid construction
of the case render the instrument capable of withstanding rough usage.
Overall dimensions are 5-1/4" × 2-5/8" × 3-1/2"; weight about 3 pounds.]
Ques. What may be substituted for the voltmeter?
Ans. A high resistance galvanometer, whose deflections are proportional
to the current, the value of the deflections being substituted in the
formula.
Ques. What precaution should be taken in making the test?
Ans. The current used should not be strong enough to appreciably heat the
resistance, and if the current be not very steady, several readings should
be taken of each measurement and the average values used in the formula.
Ques. How are the most accurate results obtained?
Ans. By selecting the known resistance as near as possible to the supposed
value of the unknown resistance.
[Illustration: Fig. 560.--Voltmeter method of testing resistances.
Knowing the resistance of the voltmeter, turn switch to the left and from
reading calculate resistance corresponding to one division of the scale.
Turn switch to right and multiply reading by resistance required for
deflection of one division. This gives resistance of voltmeter and unknown
resistance; subtracting from this the resistance of voltmeter gives value
of the unknown resistance.]
Voltmeter Method.--This is a direct deflection method and consists in
determining first the resistance that will deflect the needle through
one division of the scale on a given battery current, then with this
as a basis for comparison the voltmeter is connected across the unknown
resistance whose value is easily calculated from the reading.
In making the test, the instruments are connected as in fig. 560. The
current from battery is first passed through the galvanometer by turning
switch as shown.
[Illustration: Fig. 561.--Megohm box or set of standard high resistances.
The box contains five resistances of 200,000 ohms each. The six pillars
are petticoat insulated, the resistances being placed between each pair
of pillars. There is a double contact post on top of each pillar so that
these can be connected together with copper links.]
Assuming that the resistance of the instrument is 8,000 ohms and that
the current deflects the needle through 10 divisions of the scale,
then for a deflection of one division the resistance is
8,000 × 10 = 80,000 ohms.
Accordingly, if, when the switch is moved to the right, connecting the
voltmeter across the unknown resistance, the needle be moved through
6 divisions of the scale, the combined resistance of the voltmeter
and unknown resistance is
80,000 ÷ 6 = 13,333-1/3 ohms,
and subtracting the resistance of the voltmeter, the value of the
unknown resistance is
13,333-1/3 - 8,000 = 5,333-1/3 ohms.
Ques. For what kinds of test is the voltmeter method best adapted?
Ans. For measuring high resistances, as the insulation of wires, etc.
Ques. What may be said with respect to the current used?
Ans. Its voltage should be as high as possible within the limits of the
voltmeter scale.
[Illustration: Fig. 563.--Standard resistance box: 100,000 ohms, in four
units of 10,000, 20,000, 30,000, and 40,000 ohms. An "infinity" plug
separates each coil from the ones adjacent. Segments are elevated from
the hard rubber top by special washers in order to increase insulation.
Binding posts are so arranged as not to be in the way when plugs are used.]
[Illustration: Fig. 562.--Standard high resistance box: 100,000 ohms. It
is mounted in a brass box with a hard rubber top. Connections should be
made to terminals marked 3 and 4. When the flexible cord is on plug 1,
the box is short circuited, but when on plug 2, the resistance of 100,000
ohms is in series. The box is especially suited to rapid cable testing.]
Ques. In testing cable insulation what is desirable with respect to
voltmeter and current?
Ans. A low reading voltmeter should be used in connection with a large
battery.
[Illustration: Fig. 564.--Diagram showing principle of Wheatstone's
bridge. A, B, C, and D, are the four members which constitute the bridge.
The current from the battery divides at P, part traversing DC, and part
traversing BA. The galvanometer connected to M and N will indicate when
the currents are equal in the two branches by giving _no_ deflection.
This is then a _zero_ or _nil method_ of testing. The resistances and keys
required in testing are shown in fig. 565. In the actual instrument, the
members A, B, C, and D are known by the names given in the figure.]
Wheatstone Bridge Method.--For accurate measurements of resistance this
method is almost universally used. The so-called "Wheatstone" bridge was
invented by Christie, and improperly credited to Wheatstone, who simply
applied Christie's invention to the measurement of resistances.
[Illustration: Fig. 565.--Diagram showing arms of Wheatstone bridge,
resistances and method of connecting galvanometer, battery and unknown
resistance.]
The bridge consists of a system of conductors as shown in fig. 564. The
circuit of a constant battery is made to branch at P into two parts, which
re-unite at Q, so that part of the current flows through the point M,
the other part through the point N. The four conductors A, B, C, D, are
spoken of as the _arms_ of the balance or bridge. It is by the proportion
existing between the resistances of these arms that the resistance of
one of them can be calculated when the resistances of the other three
are known. When the current which starts from the battery arrives at P,
the pressure will have fallen to a certain value. The pressure in the
upper branch falls again to M, and continues to fall to Q. The pressure
of the lower branch falls to N, and again falls till it reaches the value
at Q. Now if N be the same proportionate distance along the resistances
between P and Q, as M is along the resistances of the upper line between
P and Q, the pressure will have fallen at N to the same value as it has
fallen to at M; or, in other words, if the ratio of the resistance C to
the resistance D be equal to the ratio between the resistance A and the
resistance B, then M and N will be at equal pressures. To find out if
this condition obtain, a sensitive galvanometer is placed in a branch wire
between M and N which will show _no_ deflection when M and N are at equal
pressure or when the four resistances of the arms "balance" one another
by being in proportion, thus:
(1) A:C = B:D
If, then, the value of A, B, and C be known, D can be calculated. The
proportion (1) is reduced to the following equation before substituting.
D = BC/A
For instance, if A and C be, as in fig. 565, 10 ohms and 100 ohms
respectively, and B be 15 ohms, D will be (15 × 100) ÷ 10 = 150 ohms.
[Illustration: Fig. 566.--Diagram showing usual arrangement of resistances
in arms of Wheatstone's bridge. In practice the bridge is seldom or never
made in the lozenge shape of the diagrams, figs. 564 and 565, these being
given merely for clearness. The resistance box of fig. 554 is, in itself,
a complete "bridge," the appropriate connections being made by screws at
various points. The letters in the above diagram correspond with those in
figs. 564 and 565, and the three figures should be carefully compared.]
As constructed, Wheatstone bridges are provided with some resistance
coils in the arms A and C, as well as with a complete set in the arm
B. The advantage of this arrangement is that by adjusting A and C, the
proportionality between B and D can be determined, and can, in certain
cases, be measured to fractions of an ohm. In fig. 565 resistances of 10,
100, and 1,000 ohms are included in the arms A and C.
[Illustration: Fig. 567.--Standard resistance box and Wheatstone bridge.
This pattern is a modification of the Anthony form of bridge. All the
resistances are wound upon metal spools. The bridge ratio coils are 1, 10,
100, 1,000, 10,000. The rheostat coils are arranged in five rows, of ten
coils each. The ordinary decade plan (explained in fig. 570) is followed.
The coils may be joined in series in multiple, or in any combination of
series and multiple. The coils may thus be checked against each other in
many combinations. For instance, all the ten ohm coils taken in parallel
may be compared with any one ohm coil. The precision of adjustment is said
to be 1/20th of 1% for the coils of the tenth ohm series, and 1/50th of 1%
for the coils of the rheostat. The ratio coils are certified to be like
each other to within 1/100th of 1%. The box is supplied with battery and
galvanometer keys of substantial construction.]
Ques. Describe the method of testing with the bridge.
Ans. Fig. 567 illustrates the general arrangement of resistances to
be found in an ordinary bridge. The connections are made as shown. In
testing, first _depress_ the battery key, then _tap_ the galvanometer key.
This should be repeated adjusting the resistances till no deflection is
obtained. The resistance then in the arm B × (C ÷ A) will give the value
of the unknown resistance.
[Illustration: Fig. 568.--Ratio coils of Wheatstone bridge. Almost every
box intended to serve as a Wheatstone bridge is furnished with a set of
coils which forms the arms of proportion or ratio arms of the bridge.
There is a choice of several different ways of arranging these coils. The
figure shows the simplest arrangement, which is employed in boxes not
intended for the highest accuracy. The required ratio, as for instance
1:100, is obtained by withdrawing a plug from each arm A and B. Ratios
1/1, 1/10, 1/100, 10/100, etc., or 1,000/1, 1,000/10, 1,000/100, 100/100,
etc., are obtainable in this manner. This simple arrangement is open
not only to the objection that the contact resistance of the plugs which
remain in is always included with the resistance unplugged, but also to
all other objections to be urged against the use of many plugs where a few
will do. The method has the limitation that it is not possible to reverse
the arms of the bridge, that is, to transpose the arms A and B.]
Ques. Why should the battery key be depressed before the galvanometer key?
Ans. To avoid the sudden swing of the galvanometer needle, which occurs
on closing circuit in consequence of self-induction.
Ques. How is it known whether too much or too little resistance be
unplugged?
Ans. The galvanometer needle will be deflected to one side for too much
resistance, and to the opposite side for too little resistance.
[Illustration: Fig. 569.--Method of reversing arms of Wheatstone bridge
with reversing blocks. The arrangement shown in the figure is classical,
being that used in the English post office type of Wheatstone bridge. It
is open to the objections which apply to the use of several plugs, one of
which is withdrawn to obtain the desired resistance.]
Ques. What is the meaning of "Inf.," marked on the bridge?
Ans. It stands for "infinity," because the resistance coil at the point
marked infinity is omitted so that adjacent sections of the arm are
disconnected when the plug is taken out.
In fact, the air gap interposed by the removal of the plug by no means
provides an infinitely great resistance, but is usually called such
because it is vastly greater than any of the other resistances of the
bridge.
[Illustration: Fig. 570.]
[Illustration: Figs. 570 and 571.--Diagrams illustrating the decade plan
of combining resistance coils. In this method the coils are connected
in series and the arrangement avoids the disadvantage of the ordinary
Wheatstone bridge in that the latter requires a large number of plugs to
short circuit the resistances not in use, which introduces an element
of uncertainty as to resistance of the plug contacts and the necessity
of adding up the values of all the unplugged resistances in order
to determine the total resistance in circuit. The necessary regular
succession of values in a rheostat built on the decade plan can be
obtained with either nine or ten coils per decade. The chief reason for
using the latter number is found in the facility with which all the coils
of one decade can be compared with one coil of the next higher decade,
thus permitting the coils of a rheostat to be checked among themselves.
Thus, the ten 1 ohm coils can be checked with a 10 ohm, the ten 10's
with a 100, etc. In some sets the ten coils of a decade can be connected
in series or in parallel, and it then becomes an advantage to have ten
coils to a decade, since the coils in one decade in parallel equal one of
the coils of the next lower decade. When these latter advantages are not
required, and especially when dials or sliding switches are used, there is
little or no advantage in using more than nine coils per decade, as shown
in fig. 570. Here all the coils of the set are connected in series so that
the circuit is never open. Thus it is a slight advantage to have permanent
connections _a_, _b_, and _c_, because all the coils of a decade can be
thrown in circuit by simply pulling out a plug, it not being necessary to
insert it again, as would be the case if the _a_, _b_, and _c_ connections
were not used. Moreover, if any plug make bad contact, its effect is
somewhat lessened by having this bad contact shunted by the remaining
coils of the decade. Again, there are occasions where violent deflections
of a galvanometer are prevented by not having the circuit entirely open
when a plug is taken out.]
The Decade Plan.--In this method of combining resistance coils, there are
9 or 10 one ohm coils for the units place, 9 or 10 ten ohm coils for the
tens place, 9 or 10 one hundred ohm coils for the hundreds place and so
on. Each series of coils of the same value is designated a _decade_. The
connections are usually made as shown in figs. 570 and 571.
It is apparent from the figure that any value in any one decade can
be obtained by inserting between a bar and a block, only one plug;
moreover if several decades be in series, any value up to the limit
of the set can be read off directly from the position of the plugs
without having to add up the unplugged resistance as in the ordinary
arrangement.
[Illustration: Fig. 572.--Two plug arrangement of ratio coils. Each of the
ratio coils has one of its terminals connected to a common center which
corresponds to the block marked C in the figure. The other terminal of
each coil is connected to an individual block, there being one block for
each coil. The bar B on one side of these blocks is joined to the rheostat
and the bar A on the other side to an X post. In the ordinary use of this
set of ratio coils two plugs only are used. One plug is inserted between
the bar A and one of the blocks, 1, 1', 10, 10', etc., of the central row
of blocks. The other plug is inserted between the bar B and any one of
the other blocks of the central row. There are two ratio coils of each
value. To obtain an even ratio as 1,000 to 1,000', one plug is inserted
between the block 1,000 and the bar A, and the other plug between the
1,000 block and bar B, the ratio arms are reversed; that is, the 1,000 ohm
coil is connected to the X post and the 1,000 ohm coil to the end of the
rheostat. When uneven ratios are used, the same ratio can be obtained by
four different combinations. To obtain the ratio one to ten, insert a plug
between A and 1, and another between B and 10, or between A and 1', and B
and 10, and get 1:10, or between A and 1, and B and 10', and get 1:10', or
again, between A and 1' and B and 10', and get 1' to 10'. Other ratios are
obtained in a similar manner. By using more than two plugs and connecting
certain of the coils in parallel combinations, a large number of other
ratios may be obtained. This arrangement offers a convenient method of
measuring the sensibility of a bridge and galvanometer combination that is
frequently applicable. If for instance the one ohm coil is used on either
side after a balance has been obtained the one ohm may be shunted with the
1,000 ohm on the same side. This will make a variation of 1/10 of 1% and
the galvanometer deflection may be noted for this variation. Similarly,
the 1 ohm may be shunted with the 100 for a variation of 1%, or with the
10,000 for a variation of 1/100 of 1%. The ten ohm coil may be shunted
with the 1,000 for a variation of 1% and with the 10,000 for a variation
of 1/10 of 1%. In the arrangement of ratio coils, errors due to plug
contacts are negligible because only two plug contacts enter the circuit,
and with an even ratio, it is only the difference in the resistances of
the two plug contacts that can affect the result. In measuring any of the
ratio coils while in the box it is only necessary to connect to the bar
C and to either the bar A or B and plug in the coils to be measured.]
[Illustration: Fig. 573.]
[Illustration: Figs. 573 and 574.--The Leeds and Northrup decade. The
object of this arrangement is to reduce the number of coils required. In
fig. 573, the 1, 3', 3 and 2 are connected in series. Let the terminals
of the 1 ohm and 2 ohm coils be numbered (1), (2), (3), (4) and (5) (fig.
573). The current enters at point (1) and leaves the coils at the point
(5), traversing 1, 3', 3, 2 = 9 ohms in all. If this series be multiplied
by any factor _n_, then _n_ (1 + 3' + 3 + 2) = _n_ 9 ohms. It will be
seen that if the points (1) and (5) be connected, all the coils are
short circuited, and the current will traverse zero resistance. If the
points (2) and (5) be connected, the 3', 3 and 2 ohm coils will be short
circuited and the current will traverse 1 ohm. By extending the process so
as to connect two and only two points at a time it is possible to obtain
the regular succession of values n (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the
last being obtained when no points are connected. Fig. 574 shows Leeds and
Northrup's method of connecting these points two at a time with the use of
a single plug. The circles in the diagram represent two rows of ten brass
blocks each. To the first two blocks at the top of the rows, the points
(5) and (1), fig. 573, are connected; to the second two, the points (2)
and (5) are connected, etc., no points being connected to the last pair
of blocks. Hence, if a plug be inserted between blocks 1 and 5, fig. 575,
the points (1) and (5) of diagram fig. 573 are connected, giving the value
of 0, if between the blocks 2 and 5 the points (2) and (5) are connected,
giving the value 1, and so on. The value 9 is obtainable when the plug is
in the last pair of blocks, which have no connections. Fig. 572 shows a
top view of the blocks of a simple decade constructed upon this plan.]
Ques. What other advantages are gained with the decade arrangement?
Ans. The single plug used with each decade is never out of use, being
either in the zero position or set on some value, and is therefore not
easily lost by being laid aside. The use of only one plug in a decade
makes it easy to ascertain that the plug is making good contact as only
one block in a row is plugged at a time, the other blocks are not kept
under a strain by having plugs forced tightly between them.
This strain on the blocks, which always exists in those sets in which
a resistance is thrown in by removing a plug, tends to separate or
loosen them and often to warp the hard rubber upon which they are
mounted. Another advantage of the decade plan is that it permits
obtaining a succession of values by means of sliding contacts or dial
switches, a method which is becoming deservedly more appreciated.
[Illustration: Fig. 575.--Leeds and Northrup dial Wheatstone bridge.
Rotating switches are used instead of plugs, which permits quicker
adjustment of the resistances, adapting it to rapid working. The ratio
coils are arranged as in fig. 568. There are four dials which form the
rheostat. The units dial contains 9 one ohm coils; the tens dial, 9 ten
ohm coils; the hundreds dial, 9 one hundred ohm coils, and the thousands
dial 9 one thousand ohm coils. The values of the ratio coils are 1, 1,
10, 10, 100, 100, 1,000, 1,000, 10,000, 10,000.]
Ques. What is the difference between "plug out" and "plug in" types of
resistance box?
Ans. In the plug out type, resistance is put in the circuit by removing
plugs, as in fig. 565; in the plug in type, resistance is put in the
circuit by inserting plugs as in figs. 570 and 571.
[Illustration: Fig. 576.--Queen Acme portable testing set. It consists
of a Wheatstone bridge, with reversible arms, battery of four dry cells,
D'Arsonval galvanometer, battery and galvanometer keys. There are sixteen
resistance coils, having a combined resistance of 11,110 ohms. Each
bridge arm is provided with three coils of 1, 10, 100 ohms, and 10, 100,
1,000 ohms respectively. The commutator admits of a ratio of 1 to 1,000
on either bridge arm, giving the set a theoretical range from .001 of an
ohm to 11,110,000 ohms. For resistances above 1,000,000 ohms, the normal
battery power must be increased. The contact keys are located as shown.
The battery key has single contact, but the galvanometer key has double
contact; depressing it closes the galvanometer circuit, and releasing it
short circuits the galvanometer, bringing the latter quickly to rest.]
Testing Sets.--For convenience in testing, a combination of the
instruments used is put up in a neat and substantial case, and known
as a testing set. There are innumerable forms of testing set, a few of
which are shown in the accompanying illustrations. The usual combination
is a Wheatstone bridge, galvanometer, battery and necessary keys and
connections.
[Illustration: Fig. 577.--Connections and circuits of Queen acme portable
testing set. There are three rows of blocks, LL', MM', NN'. LL is
connected to NN' by means of a heavy copper bar, joining L' and N'. LL'
and NN' constitute the rheostat, from which any resistance from 1 ohm
to 11,110 ohms may be obtained by removing the proper plugs. The block
N of the rheostat is connected to the lower line post D. The upper line
post C is connected to the block X of the commutator. The block C has no
other permanent connection, except key G. The block R of the commutator
is connected to the block L of the rheostat, and has no other connection,
excepting by plugs. Each half of MM' constitutes a bridge arm, designated
A and B respectively. Beginning at the lower line post D, the connections
form a continuous circuit through the rheostat, thence through the bridge
arm B, thence through the bridge arm A, thence to the upper line post
C. The commutator serves merely to reverse the bridge arms A and B. The
battery terminals are connected as shown: the positive terminal directly
to the common junction of the two bridge arms, and the negative terminal
through the battery key to the rheostat. The positive terminal of the
galvanometer is connected through the galvanometer key with the block X,
and the negative terminal with the block R of the commutator or what is
equivalent, with the block L of the rheostat. The commutator blocks A, B,
R and X, are connected by plugs as shown. When the commutator plugs are
in the position PQ, the bridge arm B is connected to the rheostat and the
bridge arm A is connected to the line, the ratio between the bridge arms
ratio being A ÷ B = X ÷ R but when the plugs are in the position ST, the
bridge arms are reversed in position A, being connected with the rheostat
and B, with the line, and the bridge arm ratio becomes A ÷ B = R ÷ X. The
connections of the testing set may be more readily understood from the
simplified diagram fig. 578.]
[Illustration: Fig. 578.--Simplified diagram showing connections of Queen
Acme portable testing set.]
Ques. Describe the operation of the Queen Acme testing set figs. 576 and
577, in measuring resistance.
Ans. Connect the terminals of the resistance to be measured to the line
posts C and D. Place the battery connections on the two upper tips 0 and
1, thus throwing one end of the battery into circuit, which is sufficient
until an approximate balance is obtained. Employ the 100 ohm coil in each
bridge arm, and place the commutator plugs in the position PQ, or in the
position ST. Then remove plugs from the rheostat until the value of total
resistance employed, or nearly as may be guessed is equal to that of the
unknown resistance. Now press the battery key Ba, and holding it down
momentarily, press the galvanometer key Ga. If the galvanometer needle
swing to the right toward the symbol + the resistance employed in the
rheostat is too high and must be reduced. If the needle swing to the left
toward -, the resistance employed is too low and must be increased. By
altering the resistance of the rheostat accordingly, a value will soon be
found, which when varied slightly either way, will reverse the deflection
of the galvanometer needle. Now remove the battery connection from tip 1,
and place it on the tip 4, thus throwing the whole battery into circuit.
Then press the keys again as before, first the battery key, then the
galvanometer key. This will increase the deflection of the galvanometer
needle for the same variation in the rheostat, thus enabling the making
of a more accurate adjustment. The measurement thus made will be the
best result that can be obtained with bridge arms of equal value, but by
selecting more suitable values of the two arms from the following table
of bridge ratios a much higher degree of accuracy may be obtained.
[Illustration: Fig. 579.--Diagram of the Queen dial decade portable
testing set. Its dimensions are 9-1/2" long, 7" wide and 7" deep, and
weighs 11-1/2 pounds. The resistances are arranged upon the dial decade
plan, being placed in circuit by means of a rotating switch contact. The
switches are so constructed that they may be turned in either direction,
thereby permitting them to be turned quickly from the highest resistance
in any dial to the lowest resistance in the same dial. This arrangement
avoids the necessity of turning back through all the remaining resistances
in any particular group of coils and is of value in locating swinging
crosses or conditions of momentary balances. The connections for the
various tests are made by the manipulation of one small knife switch
(W.B.--M.L.) and the switch BA.; these are plainly lettered, thus avoiding
the necessity of referring to a diagram of connections. In construction,
the dial switches are made up of eight laminations of No. 28 B. & S.
phosphor bronze, and the form is such as to prevent wearing grooves on
the top of the contact studs. In this instrument the electrical circuits
are soldered throughout excepting the switch contact whose resistance is
negligible. The resistances are wound with manganin. The battery comprises
six cells sub-divided which are easily replaceable. The galvanometer
is the same as in the Queen acme set, but has the addition of an Ayrton
shunt, which is useful in making insulation measurements. The necessary
keys, binding posts, and switches are provided so as to facilitate the use
of the instrument for the various measurements that can be made with it.]
Table Showing the Best Values of Bridge Arms for Measuring any
Desired Resistance
| | Position of|
| Best Values of| Commutator|
| | Plugs as|
Value of Resistance being measured | | | shown in|
| A = | B = | fig. 582|
--------------------------------------+-------+-------+------------|
Below 1.5 ohms | 1| 1,000| PQ|
Between 1.5 and 11 ohms | 1| 100| PQ|
" 11 and 78 ohms | 10| 100| PQ|
" 78 and 1,100 ohms | 100| 1,000| PQ|
" 1,100 and 6,100 ohms | 100| 100| PQ or ST|
" 6,100 and 110,000 ohms | 1,000| 100| ST|
" 110,000 and 1,110,000 ohms | 1,000| 10| ST|
" 1,110,000 and 11,110,000 ohms | 1,000| 1| ST|
Ques. In testing with the Queen Acme set how should the plugs be placed
in the commutator?
Ans. Always make the arm A the smaller except when the two arms are of
equal value.
Ques. If the resistance being measured is higher than 6,100 ohms, or lower
than 1,100 ohms, how should the commutator plugs be placed?
Ans. If higher than 6,100 ohms, they should be placed in the position ST;
if lower than 1,100 ohms, in position PQ.
When the plugs are placed in the ST position, the unknown resistance
is found by dividing the value of the larger bridge arm by that of the
smaller, and multiplying the total employed resistance in the rheostat
by the quotient. When the plugs are placed in the PQ position, the
employed resistance in the rheostat is divided by the quotient.
[Illustration: Fig. 580.--Queen portable silver chloride testing battery.
The silver chloride cell has the advantage of long life, light weight,
and compactness. The pressure of each cell when new is .8 volt.]
Direct Deflection Method with Queen Acme Set.--To measure for instance,
insulation resistance by direct deflection connect a known high
resistance, say 100,000 ohms between the line post C (fig. 577), and the
positive battery post. Remove all plugs from the commutator, and place all
plugs in the rheostat, as any employed resistance in the rheostat will
be in circuit with the galvanometer and the battery. Place the battery
connection so as to throw only one cell into circuit. Now press the keys
and obtain a deflection of the galvanometer needle. For example: assume
that the needle to be deflected about 8 divisions of the scale. Since
this deflection is due to the current from one cell passing through a
resistance of 100,000 ohms, then 100,000 × 8 = .8 megohms represents
the resistance through which one cell will produce a deflection of
one division on the scale. Hence, .8 megohms is the constant of the
galvanometer.
[Illustration: Fig. 581.--Ohmmeter. It consists essentially of a slide
wire Wheatstone bridge, with the scale divided to read either directly
in ohms, or in per cent. of a fixed resistance value. A galvanometer is
mounted on the containing case of each, and battery and galvanometer keys
are provided. In the direct reading type, the scale is so cut that when
the galvanometer is balanced, the pointer of the instrument indicates the
value of the resistance between the X posts. The scale is calibrated for
any desired range. These ohmmeters being slide wire bridges, the greatest
accuracy is at the center of the scale, hence one should be selected that
will bring the part of the scale likely to be the most used at or near the
center. A convenient type is that in which the scale is cut in per cent.,
100 per cent. being at the center of the scale. Fixed coils of 1, 10, 100,
1,000 and 10,000 ohms are contained in the instrument with a plugging
arrangement allowing any one to be used. When a balance is obtained,
the actual resistance is determined by multiplying the dial reading by
the value of the fixed coil in use. This amounts simply to shifting the
decimal point. For instance, if the 100 ohm coil were being used, and the
pointer were at .875, the resistance would be 87.5 ohms.]
Now, replace the known high resistance (100,000 ohms) by the unknown
resistance (for instance such as a cable) the value of which is to be
determined. Add enough cells to produce as large a deflection of the
needle as possible. Assume that 75 cells give a deflection of 1.5 scale
division. Then, the galvanometer constant multiplied by the number of
cells and the product divided by the deflection will give the insulation
resistance of the cable; or
0.8 megohm × 75 cells = 60.0; and
60.0 ÷ 1.5 = 40 megohms
as the resistance of the cable.
[Illustration: Fig. 582.--Commutator plug setting for comparing
electromotive forces by the fall of potential method with Queen acme set.]
Fall of Potential Method with Queen Acme Set.--To compare electromotive
forces by this method, place the battery connection (fig. 577), so as
to throw into circuit all the cells, taking care not to reverse them by
crossing the battery cords. Plug the commutator as shown in fig. 582, and
remove 1,000 ohms from bridge arm B. Place all plugs in arm A. From the
rheostat unplug 5,000 ohms. Then connect one of the cells being tested,
with its positive terminal to the + battery post and its negative terminal
to the line post C.
When the keys are pressed, the galvanometer needle will swing either to
the right or to the left. If it swing toward +, reduce the resistance
in the rheostat; if it swing toward -, add resistance to the rheostat.
When a value is found wherein a variation of an ohm either way reverses
the deflection, add to this value the resistance unplugged in arm B, and
divide the sum by the resistance in arm B. The result gives the ratio
between the voltages of the testing set battery and cell being tested
respectively. The division is decimal and may be readily accomplished by
merely pointing off as many places as there are ciphers in the resistance
employed from arm B. This operation repeated with any number of different
cells, will give their voltages in terms of the voltage of the testing
set battery, and from these ratios their relative values may be readily
obtained.
[Illustration: Fig. 583.--Diagram of apparatus for measuring low
resistances based on the principle of the Kelvin double bridge. In the
diagram AB represents a heavy piece of resistance metal of uniform cross
section and uniform resistance per unit of length; CD is another piece
of resistance metal of smaller cross section, and the two are joined
together by a heavy copper bar, AC, into which both are silver soldered;
LL are the current terminals and PP are the pressure terminals. The
resistance of AB between the marks 0 and 100 on the scale S is .001 ohm.
From the point 1 on the resistance CD to 0 on AB is also .001 ohm, from 2
to 0 is .002 and so on, and from 9 to 100 is .01 ohm. The slider M moves
along the resistance AB and its position is read on the scale S which is
divided into 100 equal parts and can be read by a vernier to thousandths.
Subdivided in this way the resistance between the tap off points PP may
have any value from .001 to .01 ohms by steps of .000001 ohm.]
If the testing set battery be replaced by a standard cell, the first
measurement gives at once the voltage of the cell tested.
If the voltage of the cell or battery being tested exceed that of the
testing set battery, reverse the position of the two batteries, and the
subsequent operations, as outlined above, will give the desired results.
How to check a Voltmeter with the Queen Acme Set.--In using a set as
in fig. 576, first remove about 10,000 ohms from the rheostat, plug the
commutator as shown in fig. 582, remove 100 ohms from the arm B, of the
bridge, and connect a standard cell with the positive terminal to the +
battery post and the negative terminal to the line post C. Then, connect
the circuit to the battery posts of the testing set the positive lead to
the + post and the negative lead to the - post. Now, press both keys and
note the direction of the deflection of the galvanometer needle. If it
move toward +, the rheostat resistance is too high; if toward -, too low.
[Illustration: Fig. 584.--Kelvin bridge. This includes a low resistance
standard of .1 ohm variable by steps of .00001 ohm, a set of ratio coils,
and a holder for rods or wires to be measured, with a scale to measure
their length. It is also provided with heavy flexibles to be used in
measuring the resistances of irregularly shaped pieces. The connections
are clearly shown in the diagram. The range of measurements of this bridge
is: 1 ohm to .1 ohm by steps of .001 ohm readily estimated to .0001; .1
to .01 ohm by steps of .0001 ohm readily estimated to .00001; .01 ohm to
.001 ohm by steps of .00001 ohm, readily estimated to .000001; .001 ohm
down by steps of .00001 ohm, readily estimated to .000001 ohm.]
Change the rheostat resistance accordingly until the balance attained is
such that a very slight variation of the rheostat resistance one way or
the other will reverse the galvanometer deflection. To find the pressure
on the circuit, add 100 to rheostat resistance and point off two places.
Multiply this value by the voltage and the product will be the desired
voltage.
If the voltage of the standard cell be exactly one volt, the total
employed resistance represents the voltage on the circuit.
[Illustration: Fig. 585.--Queen slide wire bridge. It consists of a
portable slide wire, Wheatstone bridge arranged to read directly in ohms
in addition to its use for locating crosses and grounds. It is complete
with battery, galvanometer and telephone receiver. The bridge is balanced
by moving the hand stylus until the galvanometer shows no deflection or
until there is no sound in the telephone receiver. In order to provide a
wide range of measurement and maximum accuracy, ratio coils or multipliers
having values of 1, 10, 100, 1,000 and 10,000 are provided. The scale of
the instrument is arranged in two parts, one of which indicates ohms and
the other is divided into uniform divisions for use when locating crosses
and grounds by the Murray and Varley loop methods. A small induction
coil is included so as to furnish an alternating current when using the
telephone receiver.]
For instance, in making a measurement on a 110 volt circuit, assume
that the employing of 7,840 ohms rheostat resistance produces balance,
and that increasing or decreasing this resistance by two ohms,
reverses the galvanometer deflection. This indication that the setting
7,840 is uncertain, about 1/40 of 1 per cent. Since the rheostat coils
are adjusted to an accuracy of only 1/5 of 1 per cent., that will be
about the accuracy of the measurement.
If the pressure of the standard cell be 1.018 volts, then 7,840 + 100
= 7,940. Pointing off two places, gives 79.40, which multiplied by
1.018 gives 80.82 for the voltage on the circuit.
To Measure Internal Resistance of Cell with Queen Acme Set.--First compare
its voltage on open circuit with the pressure of the testing set battery.
Then, shunt the cell with a known resistance, about 100 ohms, and again
measure its terminal voltage. The difference between the two values thus
obtained, divided by the value of the shunt resistance, will give the
value of the current. To find the internal resistance, multiply the value
of the shunt resistance by the ratio between the first and second measured
values.
[Illustration: Fig. 586.--Evershed portable ohmmeter set. This testing set
consists of a direct reading ohmmeter which indicates by direct reading
the value of the resistance being tested, also a portable hand dynamo
which provides at any required pressure the current necessary to make the
test. It is adapted to the needs of supply stations, wiring contractors
and dynamo builders. It is also useful in testing the insulation of
underground and aerial cables, and is designed so that it can be used by
ordinary workmen who are not experienced in handling delicate instruments
and who, by its use, are able to obtain accurate results. The dynamo is
wound for 100, 200, 500, or 1,000 volts, and is fitted with spring drum
inside the case on which is coiled a twin flexible cord provided with a
connector adapted for clamping under the ohmmeter terminals.]
For instance, assume that the open circuit voltage of the cell being
tested as compared with the voltage of the testing set battery is .212
of the latter, and that when it is shunted with a resistance of 1,000
ohms, its terminal voltage is .179. Then, the total resistance is to
the 1,000 ohms shunt resistance as .212 is to .179 or (.212/.179) ×
1,000 = 1,184, from which deducting the 1,000 ohms shunt resistance,
gives 184 ohms as the internal resistance of the cell.
[Illustration: Fig. 587.--Leeds and Northrup fault finder. A lineman's
instrument for the location of faults, crosses, grounds, and opens in
telephone and telegraph circuits, and for the measurement of conductor
and insulation resistance.]
[Illustration: Fig. 588.--Diagram showing arrangement and connections
of Leeds and Northrup fault finder. It is used to measure conductor
resistance, fault resistance, to locate faults by four different tests,
and when used with a buzzer and telephone, to locate opens. The essential
feature of the instrument is the uniform resistance AB, which lies in a
circle and which has a value of about 100 ohms. By a special construction,
it is so arranged that the contact can be made at any point along it, and
it is therefore equivalent to a very high resistance slide wire. It has
a moving contact C and a uniform scale of 1,000 divisions. In series with
this, there are the two resistances E and R which may be short circuited
by the switches U and V. E has exactly the same resistance as the wire AB.
R has a resistance of 100 ohms, and is the fixed resistance of the bridge
arrangement for resistance measurements. The resistances of 1,000 ohms
and 9,000 ohms connected to the battery post are to protect the battery
and the apparatus from excessive current. The 9,000 ohms may be short
circuited by the switch W.]
Ammeter Test with Queen Acme Set.--Connect a low resistance in series with
the ammeter and run leads from it to the testing set, the positive lead to
the + battery post and the negative lead to the line post C (fig. 577).
Insert a standard cell between the battery posts, with positive terminal to
+ battery post, and negative terminal to - battery post. Plug commutator
as shown in fig. 582. Remove 10,000 ohms from rheostat, and 100 ohms from
bridge arm B. Determine a balance in the usual way by changing the value
of the resistance in the rheostat. This operation will balance the
difference of pressure at the terminals of the shunt resistance against
the standard cell, and its value is equal to
(1.40 × 100) / (R + 100) = 140 / (R + 100)
To determine the current flowing, divide the value of the difference of
pressure thus obtained by the value of the shunt resistance.
[Illustration: Fig. 589.--Resistance measurement with Leeds and Northrup
fault finder. The diagram shows the proper connections and switch settings
for measuring conductor resistance. As in the ordinary slide wire bridge,
the resistance X between the two posts 1 and 2 is obtained from the
formula X = A ÷ (1,000 - A) × R. To avoid the necessity of solving in
each case the fraction A ÷ (1,000 - A), a table is furnished with the
instrument, giving the value of this fraction for each value of A. _The
resistance is accordingly determined in each case by simply setting the
contact C for a balance and reading from the table the resistance opposite
the number corresponding to the scale reading and multiplying by 100,
the value of R._ To use an outside battery, remove the inside battery and
connect the outside battery between the posts Gr and Ba. The pressure of
this battery should not exceed 110 volts. _If it exceed 25 volts, open
switch W._
EXAMPLE--With an unknown resistance connected between the posts 1 and 2,
the galvanometer showed a balance for a dial reading of 387. The number
opposite 387 in the table is .6313; hence, X = .6313 × 100 = 63.13 ohms.]
[Illustration: Fig. 590.--Diagram of the Queen standard potentiometer. The
circuit arrangement is a method of sub-dividing the main potentiometer
wire, MNOPQ, so as to provide for very accurate reading. The secondary
voltage, or that used to supply current to the main potentiometer circuit,
is adjusted by regulating rheostats, "Fast," "Medium," and "Slow" so that
the current flow is exactly .0001 ampere. It is noted that this instrument
requires a very small current for its operation. The instrument is
direct reading for voltage measurements, not exceeding 1.4+. In order to
determine if the current flow through the potentiometer be exactly .0001
ampere, the terminals of the standard cell binding posts are connected
in circuit so that the drop over points between which they are connected
are exactly equal to the voltage of the standard cell used. Binding posts
are provided for connection with various standard cells. The unknown
voltage to be measured is placed in opposition to the current flow in the
potentiometer circuit by connecting to the binding post "XEMF." Observe
that polarity is connected as required. The galvanometer with its shunt
is placed in the standard cell circuit, or X circuit, by means of a
double pole, double throw switch. The switch at T provides for standard
cells of different values and the setting at U allows for temperature
correction. The range of the instrument in volts can be increased by means
of multipliers or volt boxes.]
[Illustration: Fig. 591.]
[Illustration: Fig. 592.]
[Illustration: Fig. 593.]
[Illustration: Figs. 591 to 594.--Diagrams illustrating loop testing. To
properly understand the Murray or Varley loop tests, consider a Wheatstone
bridge (fig. 591) the arms of which are equal. In loop testing, the
rheostat is replaced by a length of cable and the unknown resistance also
by a length of cable, as in fig. 592, both being similar in resistance per
foot. If both lengths be the same, their resistances are the same and the
bridge balances. Now shorten one cable and add resistance in series with
it until the bridge again balances as in fig. 593. The added resistance
equals that of the piece cut off. Hence, if the resistance per foot be
known, the length of the shorter piece can be easily calculated. In the
Murray and Varley tests, the battery circuit is by ground connections
instead of by wire. In the Murray loop the arrangement is similar to fig.
592, the battery circuit being completed by ground connection through
fault in defective cable. Fig. 594 shows the general arrangement of the
Varley loop.]
Loop Test.--This is a method of locating a fault in a telegraph or
telephone circuit when there is a good wire running parallel with the
defective one. In the process, the good and bad wires are joined at their
distant ends and one terminal of the battery is connected to a Wheatstone
bridge, while the other terminal is grounded. There are different ways of
making loop tests as by:
1. The Murray loop;
2. The Varley loop;
3. Special loop.
[Illustration: Fig. 595.--The Murray loop test. The apparatus is connected
as in the figure. The rheostat of the bridge is used in place of the
second arm to permit large adjustment. X and Y are the resistances of the
cable between the fault and the points 1 and 2 respectively.]
The Murray Loop.--In this test only one of the two regular bridge arms
is used, the other being replaced by the rheostat giving an arm of large
adjustment.
The connections are shown in fig. 595. In making the test, close key
and note the deflection of the needle due to pressure of chemical
action at fault, if any. This is called the _false zero_.
Now apply the positive or negative pole of the battery by depressing
the battery key, and balance to the false zero previously obtained
by varying the resistance in arms A or B. Then by Wheatstone bridge
formula: RX=AY, and L=X+Y; Y=L-X, whence
X = A/(R+A)
Y = L(R/(B+A))
[Illustration: Fig. 596.--Murray loop method of fault location with Leeds
and Northrup fault finder. Case I where there are two wires having equal
resistance, in one of which there is a fault. Connect and set switches
as shown; join the good wire to post 1 and the faulty wire to post 2.
The resistance of E is equal to that of AB. From the symmetry of the
arrangement, it is evident that, if the fault were exactly at the junction
between the good and bad wires, the contact point C would rest for a
balance at 1,000 on the scale, or at 500 if the fault were half-way along
the bad wire; hence, at whatever point it comes to rest, the reading
divided by 1,000 and multiplied by the length of the bad wire is the
distance from the instrument to the fault.
EXAMPLE--In a pair of equal wires, 5.8 miles long, one is grounded. With
the connections made as above, and the galvanometer balanced for the
dial reading 124, the distance to the fault is (124 × 58) ÷ 1,000 = .7192
miles.]
[Illustration: Fig. 597.--Murray loop method of fault location with Leeds
and Northrup fault finder: Case II, where the good and bad wires are
_unequal_. The figure shows the connections. It is the ordinary Murray
loop and it is evident that the resistance _a_, to the fault will be
obtained from the formula _a_ = (A ÷ 1,000) × r, where r is the resistance
of the loop, and A is the reading of the contact C on its scale. The
distance d, to the fault is obtained from the formula d = Ar ÷ (1,000 ×
M), where M is the resistance per mile of the faulty wire.
EXAMPLE--A wire having a resistance M of 16.46 ohms per mile is grounded.
This wire was looped with a wire of unknown resistance and the total
resistance of the loop r was measured and found to be 54.07 ohms.
Connections were made as in the figure, and the reading A was found to be
332. Substituting these values in the above formula: d = (332 × 54.07) ÷
(1,000 × 16.46) = 1.09 miles.]
Ques. How may the distance from 2 to the fault be determined in knots or
miles.
Ans. Divide Y by resistance per knot or mile.
The Varley Loop.--This is a method of locating a cross or ground in a
telephone or telegraph line or other cable by using a Wheatstone bridge in
a loop formed of a good wire and the faulty wire joined at their distance
ends. One terminal of the battery is grounded and the other connected
to a point on the bridge at the junction of the ratio arms. The rheostat
arm then includes the resistance of the rheostat plus the resistance of
the fault, while the unknown arm includes the resistance of the good wire
plus the resistance of the bad wire beyond the fault. When the bridge is
balanced, the unknown resistances may be readily determined by a simple
equation.
[Illustration: Fig. 598.--The Varley loop test. The diagram shows the
various connections. X and Y are the resistances of the cable between the
fault and the points 1 and 2 respectively. L is the resistance of the good
and bad cable or X + Y.]
In making the Varley loop test, the resistance of looped cable or
conductors is measured, and then connected as in fig. 598. Close the
battery key and adjust R for balance.
When earth current is present, the best results are obtained when the
fault is cleared by the negative pole, and just before it begins to
polarize. If X be the resistance from 2 to the fault, then
X = (L - R) / 2
also, X divided by the resistance of the cable or conductor per knot
or mile gives the distance of fault in knot or miles.
When the resistance of the good wire used to form a loop with the
defective wire, together with that portion of the defective wire from
the joint to the fault is less than the resistance of the defective
wire from the testing station to the fault, the resistance R must be
inserted between point 1 and the good conductor, the defective wire
being connected directly to point. The formula in this case is
X = (L + R) / 2
[Illustration: Figs. 599 and 600.--Varley loop method of fault location
with Leeds and Northrup fault finder. This method may be used as a check
on the Murray methods. Connect the faulty wire to 1, and measure the
resistance of the loop. Then throw switches as shown in the fig. 600.
Let: a = resistance to fault, d = distance to the fault in miles, M =
resistance of the faulty wire per mile, r = resistance of the loop, R =
resistance of the coil R, or 100 ohms, T = A ÷ (1,000 - A) to be read from
the table. From the Wheatstone bridge relation: a = (r - 100T) ÷ (T + 1),
and d = (r - 100T) ÷ (T + 1)M.
EXAMPLE--A wire having a resistance of 16.46 ohms per mile is grounded.
This was looped with a wire of unknown resistance and the resistance
of the loop was found to be 54.07 ohms. Connections were made as in the
figure, and the reading A was found to be 234. From the table T = .3055,
and substituting: d = (54.07 - 30.55) ÷ (1.3055 × 16.46) = 1.094 + miles.]
Special Loop.--This method may be used to advantage where the length
of the cable or faulty wire only is known and where there are two other
wires which may be used to complete the loop. It is not necessary that
the resistance of the faulty wire and the length and resistance of the
other wires be known. Figs. 601 to 604 show the connections and method of
testing.
EXAMPLE.--All the wires in a cable 10,852 ft. long were found to
be grounded so that none of them could be used as good wires. Two
wires were selected out of another cable going to the same place by
a different route and securely joined to one of the grounded wires
at the distant end. This grounded wire and one of the good ones were
connected as shown in figs. 601 and 602 and the reading A was found
to be 307. Connections were then made as shown in figs. 603 and 604
and A was found to be 610. What is the value of d?
According to formula
d = AL/A = (307 × 10,853)/610 = 5,461 ft.
[Illustration: Figs. 601 and 602.--Special loop test with Leeds and
Northrup fault finder. For the first measurement connect the faulty wire
to 2, either of the good wires, as Z, to 1, the post Gr to ground, and
short circuit the coils R and E by closing switches U and V as in the
figures. Balance in the usual way and call the dial reading A. For the
second measurement, connect the post Gr. (disconnected from ground),
to the other good wire y as shown in figs. 603 and 604, and get another
balance; call this reading A'. The distance d, to the fault is determined
from the simple formula d = AL ÷ A' where L is the length of the cable or
faulty wire.]
[Illustration: Figs. 603 and 604.--Special loop test as made with the
Leeds and Northrup fault finder. Diagram showing connections for the
second measurement. The special loop test may be used to advantage where
the length of the cable or faulty wire only is known, and where there are
two other wires which may be used to complete the loop. To use an outside
battery, connect one pole to Ba, and ground the other. The pressure of
this battery must never exceed 110 volts; if it be over 25 volts, see that
switch W is open.]
The Potentiometer.--For the rapid and accurate measurement of voltage,
current, and resistance, the potentiometer can be recommended. Those in
charge of electric light and power companies, and also those who purchase
large amounts of electrical energy are realizing, more and more, the
necessity of having satisfactory primary standards with which to check
their volt-, ampere-, and watt-meters.
When it is realized that an error of one per cent. in a commercial
instrument means an error of one dollar one way or the other in every
one hundred dollars charged, the need of such standardization apparatus
becomes at once apparent.
The potentiometer, it should be noted, relies for its accuracy, only upon
the constancy and accuracy of resistances and upon standard cells.
With the materials now available, and the skill which has been acquired
in their manufacture, both the resistances and the standard cells are
obtainable which are remarkably constant, and both can be readily checked
for accuracy.
Location of Opens.--These measurements are based on the fact that the
capacity of wires in a cable is ordinarily a measurable quantity, which,
in wire of uniform diameter, is proportionate to length. In making these
tests, a fault finder is used together with a buzzer, dry cells to operate
it, small induction coil, and telephone receiver. These instruments are
to be found in any telephone exchange. It is best to locate the buzzer at
some distance from the fault finder in order that it cannot be heard by
the operator.
[Illustration: Fig. 605.--To use galvanometer of Leeds and Northrup fault
finder in series with the battery: Set switches as shown, and connect
between posts Gr. and 2 (see figs. 587 and 588). The galvanometer will
have the maximum sensibility with the pointer at 1,000 and the minimum at
zero.]
[Illustration: Fig. 606.--To measure high resistances, such as the
resistances of faults with Leeds and Northrup fault finder. _First
Method._--Arrange the switches as shown in the figure. Connect posts Gr.
and 2, turn the handle until the galvanometer needle comes to rest at
an even deflection of ten divisions. Call the reading A. Connect in the
unknown resistance between Gr. and 2. Now close the switch W, so that the
figure 1 appears on the top of the block, and again bring the galvanometer
to a deflection of ten divisions and call the reading B. Then X = (10,000
B ÷ A) - 1,000. In case X be a high resistance, it will be found that
the galvanometer will not deflect ten divisions for any position of the
pointer. In such case, choose a number of divisions which is a factor of
ten, such as 5, 2, or 1, and multiply (10,000 B ÷ A) by ten divided by
the number chosen, as 10/5, 10/2, 10/1. For example, for a deflection
of two divisions: X = (10/2)(10,000 B ÷ A) - 1,000. The satisfactory
range of the set for high resistance measurement may be increased by
using an outside battery of higher voltage. With the contained battery,
satisfactory measurements can be made up to 1 or 2 megohms. When outside
battery is used, connect one terminal to the post Ba, and the other to 2
for the reading A. Connect the battery and unknown resistance in series
between these posts for the reading B. When an outside pressure of 25
volts or over is used, the switch W should not be closed unless there be
a resistance in series with the battery of 10,000 ohms or over. _Second
Method._--For use as a voltmeter to measure high resistances. (More
convenient but not quite as accurate as first method.) Set the switches to
RV, M and 10. Turn the knurled nut on the galvanometer so as to set the
needle to the extreme right hand side of the scale. Connect the posts 2
and Gr. with a short piece of wire. Turn the rotating pointer on the scale
until the galvanometer needle moves over about 20 scale divisions when
the battery key is closed. Remove the connection between 2 and Gr. as the
voltmeter terminals. This makes a simple way of testing for various kinds
and amounts of trouble. On a wet cable a deflection of 10 to 15 divisions
indicates heavy enough trouble to locate with the fault finder. With a
little care, trouble showing only 5 or 6 divisions can be located.]
Before attempting locations for opens it is well to make the following
measurements:
1. The insulation of the broken wire and the insulation of the good wire
with which it is to be compared.
This may be done as shown in fig. 606. It is best that the insulation
resistance be fairly good, but experiments indicate that good results
can be obtained by the methods which follow, even when the insulation
is as low as 100,000 ohms, and fair results when as low as 50,000 to
100,000 ohms.
[Illustration: Fig. 607.--Diagram of connections in testing for opens with
Leeds and Northrup fault finder. The apparatus required consists of fault
finder, buzzer, dry cell to operate buzzer, small induction coil, and
telephone receiver. Connect the battery to the primary of the induction
coil, one terminal of the secondary to the post Ba, and the other to the
connected wires as shown. Set switches U and V so as to short circuit the
two resistance coils.]
2. The resistance between the two sections of the broken wire should be
measured.
This may be done by joining the broken wire and a good wire at the
distant end of the cable and measuring the resistance of the loop. To
ensure close locations, this resistance should be over 100,000 ohms.
Fair locations can be made when the resistance is much lower and it is
worth while to attempt it even if the resistance be as low as 10,000
ohms. The difficulty of determining the balance point increases as
the resistance decreases.
Ques. Describe the method of locating an open with a fault finder.
Ans. (_Case I_) The broken wire will be one of a pair. Select another pair
in the cable that will have the same capacity per mile and join together
the mate of the broken wire and one wire of the other pair. Make the
connections as shown in fig. 607, then depress the battery key and move
the contact to the point of minimum sound in the telephone. The distance
to the break is equal to LA ÷ (1,000 - A), where L is the length of the
good wire.
EXAMPLE: A cable 1.45 miles long contained a broken wire. It was found
that the insulation resistance of the end of this wire was over 10
megohms, as was that of the good pair selected to test against it. The
resistance between the two pieces of the good wire was also over 10
megohms. Connections were made as in fig. 607, and it was found that
the balance point was 476. Accordingly from the table
A / (1,000 - A) = 0.9084
and
d = 1.45 × .9084 = 1.317 + miles.
[Illustration: Fig. 608.--Diagram of connections in testing with Leeds
and Northrup fault finder for open wire in telegraph and other cables
in which the wires are not grouped in pairs. Connect the broken wire to
1. Select a good wire and join to 2. Connect all other wires and ground
them, by connecting to the cable sheath. Connect the distant end of the
broken wire to the others. Ground the end of the induction coil that is
not connected to the post Ba.]
Location of Opens.--(_Case II_) Open wire in telegraph or other cables in
which the wires are not grouped in pairs. The connections are made as in
fig. 608, and the measurement and calculation exactly as in the preceding
case.
The accuracy of the location of both of the above methods depends on
the good and broken pair, or the good and broken wires having equal
and uniform capacity per unit length. It is not always possible to
select wires that are alike in this respect. In such cases, as for
instance, where there is no good wire in the cable containing the
broken wire, and a good wire has to be selected from another cable,
the method of _Case III_ may be used.
[Illustration: Fig. 609.--Diagram of connections for reading in testing
for opens with Leeds and Northrup fault finder, when broken wire and good
wire are not in the same cable.]
[Illustration: Fig. 610.--Leeds and Northrup potentiometer. It is direct
reading from .000001 volt to 16 volts, and with accessories the range
may be extended to 1600 volts, and currents may be measured up to 3000
amperes. The instrument has fifteen coils of 5 ohms each, which are in
series with an extended wire about 190" long of equal resistance. The
electrical circuits are shown in the diagram fig. 611. It is well for
the user to open up the potentiometer and make himself familiar with
its interior construction, in order to fully understand the operation
of the rheostat and other parts. There are no contact resistances in
the potentiometer circuit proper. The potentiometer has low internal
resistance which gives it the maximum sensibility. Compared with high
resistance potentiometer, this is especially advantageous in measuring
the electromotive force of thermocouples, and the fall of potential
across standard low resistances. As constructed, the last one-tenth volt
is covered by the extended wire and the handle which carries the contact
point on the wire may be manipulated rapidly so that a fluctuating
voltage may be accurately followed. When used with any cadmium cell,
the potentiometer is direct reading. The accuracy of the potentiometer
resistances can be verified with the facilities of the ordinary
laboratory.]
Location of Opens.--_Case III_, in which the broken wire and good wire
are not in the same cable. Connect the good wire and broken wire in the
same way as shown in fig. 607, and set the pointer for a balance. Call the
reading A. Then connect the good wire and the broken wire at the distant
end and set the pointer for a new balance. Call this A'. The connections
for this reading are shown in fig. 609. The distance to the break will be
d = (A A' L) / (1,000(A - A') + A A')
where L is the total length of the broken wire.
[Illustration: Fig. 611.--Diagram showing connections of Leeds and
Northrup potentiometer. The coils in the series AD are each 5 ohms, and
between each two there is a brass block with a reamed hole. A pair of
flexible cords with taper plug terminals to fit these holes is furnished.
These coils can be measured with an ordinary Wheatstone bridge and thus
compared with each other to a high degree of accuracy, even if the bridge
be not accurate. For potentiometer work, the essential point is that
they should be like each other, not that they should be accurately any
particular value. In the same way the resistance of the extended wire
can be compared with the resistance of the coils in AD. Its resistance
should be 1.1 times the value of any coil between A and D. Outside
connection with the extended wire may be made by using the posts marked
BR and -BA. This adjustment for balancing an unknown electromotive force
is accomplished by the manipulation of the two contact points M and M'.
The coils AD are arranged in a circle, a revolving switch moving M. A
checking device enables the operator to set this switch without taking his
eye from the galvanometer. The resistance S is of such value that when
it shunts the wire OB, the total resistance between O and B is 1/10 of
the same unshunted. When the shunt is applied, provided the total current
remain the same, the drop between any two points on AB will be 1/10 of
its previous value. The total current will remain the same provided the
total resistance in the circuit remain the same. This is accomplished
by making the coil K such that it exactly compensates for the reduction
in resistance caused by plugging in the shunt coil S. The low scale is
applied by moving the plug from the position 1 to the position .1. With
this change the potentiometer reads from .16 volt down by indicated
steps of .000005 volt. The reading is very simple. For instance, if M
stand at 1.2 and M' at 1.35 revolutions, the reading is 1.2135 volts.
The resistances of the instrument are wound upon metal spools, and are
therefore able to dissipate a comparatively large amount of energy. This
allows the potentiometer to be used for pressure measurements up to 16
volts without the use of a volt box.]
[Illustration: Fig. 612.--Diagram showing actual connections in the
rheostat of Leeds and Northrup potentiometer. The figure corresponds to
R of fig. 611. The rheostat is mounted in the end of the potentiometer
as shown in fig. 610. Rough adjustment of the potentiometer current is
made by means of the variable resistance R. Fine adjustment is made by
means of the variable resistance R'. It will be noted that the 23 ohm
resistance of this latter rheostat is shunted by a resistance of 6.1 ohms,
making possible a very fine regulation. Further, there is in series with
the moving contact a resistance of 400 ohms, which makes the effect of
variable contact resistance negligible. Only one cell of storage battery
should be used. When this battery is fresh, the plug shown in the figure
at 2R should be inserted at R. This gives the greatest resistance in the
rheostat circuit. As the cell runs down, the plug should be changed to
2R. When both plugs are in, the rheostat slide wires are in series with
the potentiometer circuit.]
EXAMPLE: A pair of wires containing one broken wire was connected with
a good pair in a different cable as shown in fig. 607. The reading A
was found to be 180. The good and bad wires were then joined at the
distant end as in fig. 609, and the reading A was found to be 88. The
total length of the bad wire MN was 1.44 miles. Required, the distance
to the break. Substituting the values in the formula:
d = 180 × 88 × 1.44 / 1,000(180 - 88) + 180 × 88 = .211 + mile.
To Pick Out Faulty Wires in a Cable.--Short circuit the coils E and R with
switches U and V. Set the pointer at 1,000. Connect the post Gr. to ground
or the cable sheath and apply the wires one after another to the binding
post 2. The galvanometer will deflect for a faulty wire.
[Illustration: Fig. 613.--Diagram of the Crompton potentiometer. In this
instrument the resistance consists of fourteen coils, each of 10 ohms,
in series with a straight wire, also 10 ohms resistance, thus forming a
system of fifteen equal steps. Across the whole a pressure of 1.5 volt
is applied from a secondary cell, thus providing .10 volt per step. Any
fraction is then tapped off by means of a radial switch on the resistance
coils and a sliding contact on the wire. The standardization is performed
by adjusting a resistance in series with the whole until the standard cell
employed indicates, by means of the galvanometer G, a balance at the point
which represents its electromotive force on the basis given above.]
Ques. What is a potentiometer?
Ans. An arrangement of carefully standardized resistances for measuring
voltages in comparison with a standard cell. It is used for accurate
measurement of voltages, currents, and resistances.
In place of a series of standardized resistances, a slide wire may be
used as in fig. 614.
Ques. Describe one form of potentiometer.
Ans. As shown in fig. 614, it consists of a fine German silver wire about
3 feet long stretched between the binding posts A, B, which are attached
to a wooden base carrying a scale divided into 1,000 equal parts. There
are three circuits, the terminal A being included in each, one including
the battery, and the other two the galvanometer. A three point switch
connects the galvanometer in series with the standard cell SC, or the cell
to be tested C, the circuits being completed by leads terminating in the
sliding contacts M and S.
[Illustration: Fig. 614.--Diagram of potentiometer showing method of
measuring the voltage of a cell. The potentiometer is simply a high
resistance wire of uniform diameter stretched between two binding posts,
A and B, in such a way that contact can be made at its ends and along
its length. Necessary circuits are plainly shown in the figure; SC, is
a _standard cell_ and C, the cell to be tested. M, and S are sliding
contacts, connecting with the "slide wire."]
Ques. Describe the method of measuring the voltage of a cell with a
potentiometer.
Ans. Fig. 614 shows a method of comparing a pressure with that of a
standard cell and is applicable whether the pressure of the cell to
be tested be greater or less than that of the standard cell. In making
the test the switch F is first closed, then the other switch is moved
to D, and M adjusted till galvanometer shows no deflection; similarly,
the switch is moved to G, and S adjusted till galvanometer shows no
deflection. Then, C:SC = AS:AM. from which C = SC × AS ÷ AM.
EXAMPLE.--Let 1.016 volts be the known voltage of the standard cell
SC, and the scale reading of AS be 657, and of AM, 225 as in the
figure, then
C = (1.016 × 657) / 225 = 2.966 volts
The arrangement may, however, be made direct reading, that is,
the slide wire may have a scale of volts instead of lengths or
resistances, as follows: Suppose the standard cell to have a pressure
of 1.434 volts, the sliding contact M is placed at the reading 1.434,
and the adjustable resistance varied till the galvanometer shows no
current. This means that the pressure between A and M is 1.434, and
consequently the pressures all along the slide can be read off the
scale _in volts_. Hence, when S has been adjusted to balance, the
pressure of C is read off the scale in volts.
[Illustration: Fig. 615.--To measure a pressure greater than 1.6 volts
with Leeds and Northrup potentiometer by using a volt box or multiplier.
To measure high voltages it is necessary to connect the voltage across
high resistance and to measure on the potentiometer a definite fraction
of the total drop. In the figure, AB is a high resistance of which CB is
.1th, DB .01th, and EB .001th of the total resistance. The potentiometer
reading is accordingly multiplied by 10, 100, or 1,000, depending upon
whether the switch M is set on C, D, or E. Resistance boxes for this
purpose are called _volt boxes_, and are constructed to multiply the
potentiometer readings by 10, 100, and 1,000. In using them, it is only
necessary to connect the unknown E.M.F. at the posts so marked, and the
potentiometer to the posts marked P. The potentiometer reading is taken
as above and multiplied by a factor depending upon the position of the
switch M, which factor is indicated upon the box. _It is essential in
making these connections that the polarity be carefully observed._]
How to Use a Potentiometer.--All connections must be made as indicated by
the stamping on the instrument. Particular attention must be given to the
polarity of the standard cell, of the battery, and of the voltage, the
corresponding + and - signs being marked. If used with a wall galvanometer
having a telescope and scale, it will be found convenient to place the
potentiometer so that the telescope is directly over the glass index of
the extended wire, thus permitting the observer to read the galvanometer
deflections and potentiometer settings without changing his position.
Potentiometer Current.--A medium sized storage cell will be found
desirable, producing a steady current. Errors in measurements are
frequently made by using an unsteady current.
Setting for Standard Cell.--Set the standard cell to correspond
with the certified pressure of the standard cell as given in its
certificate. In using the potentiometer shown in fig. 610, place
the plug in hole 1, and see that it is always in this position when
checking against the standard cell. Place the double throw switch at
STD. CELL.
Adjust the regulating rheostat until the galvanometer shows no
deflection. In making the first adjustment use the key marked R_{1};
when a balance is almost attained, use key R_{2}, and for the final
adjustment use key marked R_{0}. This cuts out the resistance in
series with the galvanometer and gives the maximum sensibility.
Measurement of Unknown Pressure.--The potentiometer (fig. 610),
as ordinarily used, gives direct readings for voltages up to and
including 1.6 volts. For pressures higher than 1.6 volts, a volt
box or multiplier should be used. After obtaining the standard cell
balance, as previously described, place the double throw switch in
the position marked E.M.F. The balance for the unknown E.M.F. is
obtained by manipulating the tenths switch and rotating the contact
on the extended potentiometer wire. The final position of the two
contacts in conjunction with the position of the plug at the left of
the instrument indicates the voltage under test.
As directed above, use key R_{1} for rough adjustment, R_{2} for
intermediate adjustment, and key R_{0} for final adjustment.
Plug at 1 gives readings for voltage directly from settings of tenths
switch and extended wire contact.
Plug at .1 shunts the potentiometer circuit so that the voltage
measured is .1 of the reading taken directly from the scale. Hence,
the readings taken from the setting of the tenths switch and the slide
wire contact must be divided by 10.
To Balance Galvanometer for Unknown Voltage.--Place plug in hole 1
(fig. 610) for voltages up to 1.6, and in hole .1 for voltages up
to .16. Rotate the tenths switch until a condition of balance is
obtained exactly or approximately. To secure an exact balance, rotate
the contact on the extended wire. The unknown voltage can now be read
directly from the position of the tenths switch and the extended wire
contact if plug be at 1, or by dividing by 10 if plug be at .1.
EXAMPLE.--A balance was obtained with the tenths switch at 1.3, the
extended wire contact at 176 and the plug at 1. The voltage under
test, therefore, is 1.3176. If the plug at .1 had been used, the same
reading would have indicated .13176.
To ascertain if the current in the potentiometer circuit has altered
during a measurement, it is only necessary to plug in at 1, place
the double throw switch on STD. CELL and close the galvanometer key.
No deflection indicates that the current has not changed. If the
galvanometer deflect, the regulating rheostat must again be adjusted
until the galvanometer shows no deflection.
To Measure Voltages from 1.6 to 16.--Pressures up to 16 volts may
be measured by using a greater voltage across the BA posts (fig.
615). For this purpose a battery of about 20 volts should be used.
Insert the large plug at .1 and throw the switch to STD. CELL, then
balance the galvanometer by means of the regulating rheostat. When the
rheostat has been set to secure a balance, insert the large plug at
1, set the switch on E.M.F. and read the voltage in the usual manner.
Multiply the reading by 10.
[Illustration: Fig. 616.--Measurement of current with potentiometer. This
is done by measuring the drop in volts across a known low resistance.
In the figure S is the standard resistance, and on it are the pressure
terminals pp, and the current terminals CC. The potentiometer is connected
to the shunt through the posts marked P. The resistance between the
points pp is adjusted to an even fraction of an ohm. These resistances
are so chosen that in order to determine the current passing through
the shunt, after having obtained a potentiometer balance it is only
necessary to multiply the potentiometer reading by a simple factor. For
instance, in using a .01 ohm standard. It is only necessary to multiply
the potentiometer reading by 100, which gives the current reading in
amperes; similarly, a .1 ohm requires multiplication by 10, and a .001
ohm by 1,000.]
Care of Potentiometer.--The slide wire, although protected to a great
extent by the hood, in time accumulates dust and dirt with a thin film
of oxide. This will tend to increase the resistance in this part of the
circuit owing to poor contact. This wire should, therefore, be cleaned
occasionally.
To do this, unscrew the stop against which the hood strikes when
turned to read zero; then remove the hood and rub the entire slide
wire vigorously with a soft cloth dipped in vaseline. _Do not use
emery or sand paper as this will destroy the uniformity of the slide
wire._ Clean also the steel contact which rubs on the wire, as this
becomes glazed after much use. When the potentiometer is not in use,
the hood should be screwed all the way down, and the lid put in place
to exclude dust.
If it be used in a chemical factory, laboratory, or any place where
acid fumes are prevalent, this latter precaution is important, because
the fumes may attack the slide wire.
It is also well to keep the contact surfaces of the switch studs clean
and bright by wiping them occasionally with a soft cloth dipped in
vaseline.
[Illustration: _Res. of leads on each end is equal to 10 divisions of
slide wire. The slide wire is divided into 1000 parts--20 for the leads,
or 980 divisions. Calibrated scale on Galvanometer:_
Fig. 617.--Diagram of Leeds and Northrup bridge for locating faults in
power circuits, showing arrangement of the connections including the lead
cables and galvanometer contacts. Make connections as shown. The clamps
must be so fastened at A and C that the contact resistances will be very
small. This contact resistance will figure as an error in the measurement.
If, for instance, the contact resistance were equal to .001 of an ohm,
and the wire were of such a size that .001 of an ohm were equal to the
resistance of 20 feet of the cable, there would be an error of 20 feet
in the location of the fault. For this reason all contact resistances
throughout the loop from A to C must be extremely small. The battery is
to be connected to the posts marked Ba., and the post marked Gr. is to
be grounded. It will very frequently happen that the ground is to the
cable sheath or some other conductor. In this case, the binding post Gr.
should be grounded to this conductor. Sufficient battery should be used
to give a readable deflection on the galvanometer for a small movement of
the contact on the bridge wire. The fault is located by the usual Murray
formula. If, for instance, the galvanometer show no deflection when the
contact is at 300 on the scale, it would indicate that the fault is at a
distance from A equal to .003 of the total length of the loop from A to C.
A testing current of five amperes may be used with this bridge. In cases
of necessity, this current may be increased to eight amperes, but when
this current is used it should not be allowed to pass through the bridge
for a longer time than is necessary. It frequently happens that small
faults which have a very high resistance develop in high pressure cables.
Such faults are likely to break down and result in damage and should be
located. It is usually impossible to locate these faults until they have
been partially carbonized. This must be done by applying a sufficiently
high voltage between the cable and the sheath (or whatever it is grounded
on) to break down the fault. In order to prevent the breaking down process
from resulting in a serious burn out a high resistance must be placed in
the circuit which will prevent an excessive current, or the circuit must
be carefully fused. The former procedure is the better.]
Location of Faults where the Loop is Composed of Cables of Different Cross
Sections.--Faults in loops of this character may be located with the same
degree of accuracy as those in loops of a uniform cross section, provided
the length and cross section of each length of cable are known. An example
will illustrate the method:
In the diagram, fig. 617, assume the length of the cable AE to be
550 yards of 25,000 cir. mil., EF, 500 yards of 40,000 cir. mil., and
FC, 1,050 yards of 30,000 cir. mil. These lengths must be reduced by
calculation to equivalent lengths of one size, and for this purpose it is
best to select the largest size. The results of this calculation are as
follows:
550 yds. of 25,000 cir. mil. = 880 yds. of 40,000 cir. mil.
500 " " 40,000 " " = 500 " " 40,000 " "
1,050 " " 30,000 " " = 1,400 " " 40,000 " "
This makes the total resistance of the loop equivalent to 2,780 yards
of 40,000 cir. mil. If the contact show a balance for a reading of
372.5, this indicates that the fault is at a distance of 372.5/1,000 of
2,780 = 1,035.5 equivalent yards. Of this, 880 are in the stretch A E.
Consequently the fault is:
1,035.5 - 880 = 155.5 yards from E.
[Illustration: Fig. 618.--The Fischer portable cable testing set, designed
for locating crosses, grounds and breaks in cables, also for conductor
and liquid resistance measure. The distinguishing feature of the set is
the _master switch_. By means of this switch, connections can be made for
the various tests by a single movement, thus avoiding the labor and time
which have to be expended in interchanging the connections and memorizing
the rather complicated scheme of connections.]