HOW TO DETECT AND MEASURE ELECTRICITY
MEASURING INSTRUMENTS.--The production of an electric current would not
be of much value unless we had some way by which we might detect and
measure it. The pound weight, the foot rule and the quart measure are
very simple devices, but without them very little business could be
done. There must be a standard of measurement in electricity as well as
in dealing with iron or vegetables or fabrics.
As electricity cannot be seen by the human eye, some mechanism must be
made which will reveal its movements.
THE DETECTOR.--It has been shown in the preceding chapter that a current
of electricity passing through a wire will cause a current to pass
through a parallel wire, if the two wires are placed close together, but
not actually in contact with each other. An instrument which reveals
this condition is called a _galvanometer_. It not only detects the
presence of a current, but it shows the direction of its flow. We shall
now see how this is done.
For example, the wire (A, Fig. 35) is connected up in an electric
circuit with a permanent magnet (B) suspended by a fine wire (C), so
that the magnet (B) may freely revolve.
[Illustration: _Fig. 34._ _Fig. 35._ _Fig. 36._
TO THE RIGHT, COMPASS MAGNET, TO THE LEFT]
For convenience, the magnetic field is shown flowing in the direction of
the darts, in which the dart (D) represents the current within the
magnet (B) flowing toward the north pole, and the darts (E) showing the
exterior current flowing toward the south pole. Now, if the wire (A) is
brought up close to the magnet (B), and a current passed through A, the
magnet (B) will be affected. Fig. 35 shows the normal condition of the
magnetized bar (B) parallel with the wire (A) when a current is not
passing through the latter.
DIRECTION OF CURRENT.--If the current should go through the wire (A)
from right to left, as shown in Fig. 34, the magnet (B) would swing in
the direction taken by the hands of a clock and assume the position
shown in Fig. 34. If, on the other hand, the current in the wire (A)
should be reversed or flow from left to right, the magnet (B) would
swing counter-clock-wise, and assume the position shown in Fig. 36. The
little pointer (G) would, in either case, point in the direction of the
flow of the current through the wire (A).
[Illustration: _Fig. 37._ INDICATING DIRECTION OF CURRENT]
SIMPLE CURRENT DETECTOR.--A simple current detector may be made as
follows:
Prepare a base 3' × 4' in size and 1 inch thick. At each corner of one
end fix a binding post, as at A, A', Fig. 37. Then select 20 feet of No.
28 cotton-insulated wire, and make a coil (B) 2 inches in diameter,
leaving the ends free, so they may be affixed to the binding posts (A,
A'). Now glue or nail six blocks (C) to the base, each block being 1" ×
1" × 2", and lay the coil on these blocks. Then drive an L-shaped nail
(D) down into each block, on the inside of the coil, as shown, so as to
hold the latter in place.
[Illustration: _Fig. 38._ THE BRIDGE]
Now make a bridge (E, Fig. 38) of a strip of brass 1/2 inch wide, 1/16
inch thick and long enough to span the coil, and bend the ends down, as
at F, so as to form legs. A screw hole (G) is formed in each foot, so it
may be screwed to the base.
Midway between the ends this bridge has a transverse slot (H) in one
edge, to receive therein the pivot pin of the swinging magnet. In order
to hold the pivot pin in place, cut out an H-shaped piece of sheet brass
(I), which, when laid on the bridge, has its ends bent around the
latter, as shown at J, and the crossbar of the H-shaped piece then will
prevent the pivot pin from coming out of the slot (H).
[Illustration: _Fig. 39._ DETAILS OF DETECTOR]
The magnet is made of a bar of steel (K, Fig. 39) 1-1/2 inches long, 3/8
inch wide and 1/16 inch thick, a piece of a clock spring being very
serviceable for this purpose. The pivot pin is made of an ordinary pin
(L), and as it is difficult to solder the steel magnet (K) to the pin,
solder only a small disc (M) to the pin (L). Then bore a hole (N)
through the middle of the magnet (K), larger in diameter than the pin
(L), and, after putting the pin in the hole, pour sealing wax into the
hole, and thereby secure the two parts together. Near the upper end of
the pin (L) solder the end of a pointer (O), this pointer being at right
angles to the armature (K). It is better to have a metal socket for the
lower end of the pin. When these parts are put together, as shown in
Fig. 37, a removable glass top, or cover, should be provided.
This is shown in Fig. 40, in which a square, wooden frame (P) is used,
and a glass (Q) fitted into the frame, the glass being so arranged that
when the cover is in position it will be in close proximity to the upper
projecting end of the pivot pin (L), and thus prevent the magnet from
becoming misplaced.
[Illustration: _Fig. 40._ CROSS SECTION OF DETECTOR]
HOW TO PLACE THE DETECTOR.--If the detector is placed north and south,
as shown by the two markings, N and S (Fig. 37), the magnet bar will
point north and south, being affected by the earth's magnetism; but when
a current of electricity flows through the coil (B), the magnet will be
deflected to the right or to the left, so that the pointer (O) will then
show the direction in which the current is flowing through the wire (R)
which you are testing.
The next step of importance is to _measure_ the current, that is, to
determine its strength or intensity, as well as the flow or quantity.
DIFFERENT WAYS OF MEASURING A CURRENT.--There are several ways to
measure the properties of a current, which may be defined as follows:
1. THE SULPHURIC ACID VOLTAMETER.--By means of an electrolytic action,
whereby the current decomposes an acidulated solution--that is, water
which has in it a small amount of sulphuric acid--and then measuring the
gas generated by the current.
2. THE COPPER VOLTAMETER.--By electro-chemical means, in which the
current passes through plates immersed in a solution of copper sulphate.
3. THE GALVANOSCOPE.--By having a coil of insulated wire, with a magnet
suspended so as to turn freely within the coil, forming what is called a
galvanoscope.
4. ELECTRO-MAGNETIC METHOD.--By using a pair of magnets and sending a
current through the coils, and then measuring the pull on the armature.
5. THE POWER OR SPEED METHOD.--By using an electric fan, and noting the
revolutions produced by the current.
6. THE CALORIMETER.--By using a coil of bare wire, immersed in paraffine
oil, and then measuring the temperature by means of a thermometer.
[Illustration: _Fig. 41._ ACID VOLTAMETER]
[Illustration: _Fig. 42._ COPPER VOLTAMETER]
7. THE LIGHT METHOD.--Lastly, by means of an electric light, which
shows, by its brightness, a greater or less current.
THE PREFERRED METHODS.--It has been found that the first and second
methods are the only ones which will accurately register current
strength, and these methods have this advantage--that the chemical
effect produced is not dependent upon the size or shape of the apparatus
or the plates used.
HOW TO MAKE A SULPHURIC ACID VOLTAMETER.--In Fig. 41 is shown a simple
form of sulphuric acid voltameter, to illustrate the first method. A is
a jar, tightly closed by a cover (B). Within is a pair of platinum
plates (C, C), each having a wire (D) through the cover. The cover has a
vertical glass tube (E) through it, which extends down to the bottom of
the jar, the electrolyte therein being a weak solution of sulphuric
acid. When a current passes through the wires (D), the solution is
partially decomposed--that is, converted into gas, which passes up into
the vacant space (F) above the liquid, and, as it cannot escape, it
presses the liquid downwardly, and causes the latter to flow upwardly
into the tube (E). It is then an easy matter, after the current is on
for a certain time, to determine its strength by the height of the
liquid in the tube.
HOW TO MAKE A COPPER VOLTAMETER.--The second, or copper voltameter, is
shown in Fig. 42. The glass jar (A) contains a solution of copper
sulphate, known in commerce as blue vitriol. A pair of copper plates
(B, B') are placed in this solution, each being provided with a
connecting wire (C). When a current passes through the wires (C), one
copper plate (B) is eaten away and deposited on the other plate (B'). It
is then an easy matter to take out the plates and find out how much in
weight B' has gained, or how much B has lost.
In this way, in comparing the strength of, say, two separate currents,
one should have each current pass through the voltameter the same length
of time as the other, so as to obtain comparative results.
It is not necessary, in the first and second methods, to consider the
shapes, the sizes of the plates or the distances between them. In the
first method the gas produced, within a given time, will be the same,
and in the second method the amount deposited or eaten away will be the
same under all conditions.
DISADVANTAGES OF THE GALVANOSCOPE.--With the third method (using the
galvanoscope) it is necessary, in order to get a positively correct
reading instrument, to follow an absolutely accurate plan in
constructing each part, in every detail, and great care must be
exercised, particularly in winding. It is necessary also to be very
careful in selecting the sizes of wire used and in the number of turns
made in the coils.
This is equally true of the fourth method, using the electro-magnet,
because the magnetic pull is dependent upon the size of wire from which
the coils are made and the number of turns of wire.
OBJECTIONS TO THE CALORIMETER.--The calorimeter, or sixth method, has
the same objection. The galvanoscope and electro-magnet do not respond
equally to all currents, and this is also true, even to a greater
extent, with the calorimeter.