Profs. Ayrton and Perry lately described and exhibited before the
Physical Society their new ammeters and voltmeters, also a non-sparking
key. The well known ammeters and voltmeters of the authors used for
electric light work are now constructed so as to dispense with
a constant, and give the readings in amperes and volts without
calculation. This is effected by constructing the instruments so
that there is a falling off in the controlling magnetic field, and a
considerable increase in the deflecting magnetic field. The deflections
are thus made proportional to the current or E.M.F. measured. The
ingenious device of a core or soft iron pole-piece, adjustable between
the poles of the horseshoe magnet, is used for this purpose. By means of
an ammeter and voltmeter used conjointly, the resistance of part of the
circuit, say a lamp or heated wire, can be got by Ohm's law. Profs.
Ayrton and Perry's non-sparking key is designed to prevent sparking with
large currents. It acts by introducing a series of resistance coils
determined experimentally one after the other in circuit, thereby
cutting off the spark.
* * * * *
NEW INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS AND ELECTRO-MOTIVE
FORCE.
By Messrs. R. E. CROMPTON and GISBERT KAPP.
[Footnote: Paper read before the Society of Telegraph Engineers, 14th
February, 1884.]
In consequence of the rapid development of that part of electrical
science which may be termed "heavy electrical engineering," reliable
measuring instruments specially suitable for the large currents
employed in lighting and transmission of energy have become an absolute
necessity. As usual, demand has stimulated supply, and many ingenious
and useful instruments have been invented, the manufacture of which
forms at the present day an important industry. Mr. Shoolbred, in a
paper which he recently read before this Society, gave a full and
interesting account of the labors of our predecessors in this field.
To-day we add to the list then given a class of instruments invented by
us, examples of which are now before you on the table. We have preferred
to call them current and potential indicators in preference to meters,
considering that the latter term, or rather termination, ought to be
applied rather to integrating instruments, which the necessities of
electric lighting, we believe, will soon bring into extensive use. The
principal aim in the design of these indicators has been to obtain
instruments which will not alter their calibration in consequence of
external disturbing forces. If this object can be attained, then it will
be possible to divide the scale of each instrument directly into amperes
or volts, as the cause may be, and thus avoid the use of a coefficient
of calibration by which the deflection has to be multiplied. This is an
important consideration when it is remembered that in many cases
these instruments have to be used by unskilled workmen, to whom a
multiplication involving the use of demical fractions is a tedious and
in some cases even an impossible task.
[Illustration: FIG. 1. FIG. 2.]
All measurements are comparative. We measure weights or forces by
comparison with some generally known and accepted unit standard
weights, lengths, areas, and volumes, by comparison with a unit length,
resistance by a standard ohm, and so forth. In the same way currents
could be measured by comparison with a standard current: but this
would be a troublesome process, not only on account of the apparatus
necessary, but also because it would be a matter of some difficulty to
have a standard current always ready for use. In general, measurement
by direct comparison with a standard unit is discarded for the more
indirect method of measuring not the current itself, but its chemical,
mechanical, or magnetic effect. The chemical method is very accurate if
a proper density of current through the surface of the electrodes be
used,[1] but since it requires a considerable time, and, above all, an
absolutely constant current, its use is almost entirely restricted
to laboratory work and to the calibration of other instruments. For
practical ready use, instruments employing the mechanical or magnetic
effect of the current are alone suitable. We weigh, so to speak, the
current against the force of a magnet, of a spring, or of gravity.
The measurement will be exact if the thing against which we weigh or
counterbalance the current itself retains its original standard value.
Where permanent magnets or springs are used as a balancing force, this
condition of constancy in our weights and measures is not always fully
maintained, and to make matters worse, there is no visible sign by which
a change, should it have occurred, can be readily detected. A spring
may have been overstrained or a steel magnet may have become weakened
without showing the least alteration in outward appearance. To overcome
this difficulty, the obvious remedy is not to use springs or steel
magnets at all, but to substitute for these some other force which
should be either absolutely constant, such as the force of gravity, or
at least should, vary only within narrow limits, and this in accordance
with a definite law. This latter condition can be fulfilled by the
employment of electro-magnets.
[Footnote 1: According to recent experiments made by Dr. Hammerl, the
density of current in a copper voltameter should be half an ampere per
square inch of surface.]
[Illustration: FIG 3.]
To imitate with an electro magnet as nearly as possible a permanent
magnet, so that the former can be used to replace the latter, it is
necessary that the magnetism in the iron core should remain constant.
This could, of course, be done by exciting the electro magnet with a
constant current from a separate source. (In a recent note to the Paris
Academy of Science, M.E. Ducretet described a galvanometer with steel
magnet, which is surrounded by an exciting coil. When recalibration
appears necessary, a known standard current from large Daniell cells is
sent through this coil during a certain time, and thus the magnet is
brought back to its original degree of saturation. M. Ducretet also
mentions the use of a soft iron bar instead of a steel magnet, in which
case the current from the Daniell cells must be kept on during the time
an observation is taken.) But such a system would appear to be too
complicated for ready use. Moreover, some sort of indicator would be
required by which we could make sure that the exciting current has the
normal strength.
[Illustration: FIG 4.]
The plan we adopt is to excite the electro magnet by the whole or a part
of the current which is to be measured. Since this current varies, the
power exciting the core of the electro magnet must also vary; and since
we require the core to have as nearly as possible a permanent magnetic
force, we are brought face to face with the question, whether an electro
magnet can be constructed that has a constant moment under varying
exciting currents. This question has been answered by the well known
experiments of Jacobi, Dub, Mueller, Weber, and others. To get an
absolutely constant magnetic moment, is not possible, but between
certain limits we can get a very near approximation to constancy.
[Illustration]
The relation between exciting power and magnetic moment is very
complicated, depending not only on the dimensions and shape of the core
and the manner of winding, but also on the chemical constitution of the
iron of the core. It is not possible, or at least it has hitherto not
been found possible, to embody all these various elements into an exact
mathematical formula, which would give the magnetic moment as a function
of the exciting current; but the above mentioned experiments have shown
that within certain limits, and in the neighborhood of the point of
saturation, the relation between the two is that of an arc to its
geometrical tangent. It will be seen that for large angles the arc
increases much slower than the tangent; that is, for strongly excited
cores, a very large increase of the exciting current will produce only
a slight increase of magnetic moment. If Mueller's formula were correct
for all currents, absolute saturation could only be reached with an
infinite current. Whether this be the case or not, it is certain that
the greater the exciting current the less will a variation in it affect
the magnetic moment of the core. To imitate as nearly as possible
permanent steel magnets, it is therefore necessary to use electro
magnets, the cores of which are easily saturated. The core should be
thin and long and of the horseshoe type; the amount of wire wound round
it should be large in comparison with the size of the core.
[Illustration]
Here is a magnet partly wound which was used in one of our earliest
experiments, but which was a failure on account of having far too much
mass in the core in comparison with the amount of copper wire wound
round it. Since then we have greatly diminished the iron and increased
the copper. The cores of the instruments on the table are composed of
two or three No. 18 b.w.g. charcoal iron wires, and are wound with one
layer of 0'120 inch wire in the case of the current indicators, and
eighteen layers of 0.0139 inch wire in the case of the potential
indicator. If from the diagram, Fig. 1, we plot a curve the abscissae of
which represent exciting current, and the ordinates magnetic moment of
the soft iron core, we find that a considerable portion of the curve
is almost a straight and only slightly inclined line. If it, were a
horizontal straight line the core would be absolutely saturated, but
such as it is, it answers the purpose sufficiently well, for with a
variation of exciting current from 10 to 100 amperes the magnetic moment
varies but slightly. If a small soft iron or magnetic steel needle, _n
s_, be suspended between the poles, S N, of an electro magnet of such
proportions as described above, and the current, after exciting the
electro magnet, _e e_, be lead round the coils, DD, it will be found
that for all currents between 10 and 100 amperes the needle, _n
s_, shows a definite deflection for each current. Here we have a
galvanometer with permanent calibration. In this case the deflection of
the needle will not strictly follow the law of tangents, because the
directing power of the electro magnet is not absolutely constant; but
whatever the exact ratio between deflection and current may be, it must
always remain the same, and to each angle of deflection corresponds one
definite strength of current.
[Illustration]
The force with which the electro magnet tends to keep the needle in its
zero position, that is, in line with the poles, S N, is due partly to
the magnetism of the core, which is nearly constant, and partly to the
magnetic influence of the coils, _ee_, themselves, which is, of course,
simply proportional to the current. The total magnetic force acting on
the needle is, therefore, represented by the sum of these two forces,
and consequently not nearly so constant as might be desired in order to
get a good imitation of a tangent galvanometer with a permanent magnet.
In the diagram, Fig. 2, the curve, O A B, represents the magnetic moment
of the iron core, the straight line, ODE, that of the exciting coils per
se, and the dotted line, O F M, the sum of the two, obtained by adding
for every current, O C, the respective ordinates, CD and C A.