If no external retarding force were applied to the armature of such an instrument, it would tend to run at an excessive speed for any current flowing through the apparatus and to make the instrument give an accurate record of power, a retarding force which is propor tional to the speed of the armature must be applied to the spindle. This is admirably ar ranged by placing at the bottom of the spindle S a flat disk of aluminum C, on either side of which are placed the poles of permanent mag nets M. The rotation of the disc between the magnet poles generates electric currents in it which are attracted by the magnets and retard the motion of the disc.
by the frequency of the alterations of the cur rent. This is brought about by introducing a coil of high resistance and of practically no self-induction in series with the fine wire mov ing coil of the instrument.
Another form, called an ((induction watt hour meter) is usually used for recording the watthours in alternating current circuits. This instrument in its simplest form illustrated in Fig. 12, consists of coarse wire coils F F and a fine wire coil C connected to the circuit as in a wattmeter. The iron core of the fine wire coils is adjusted so that the magnetic flux lags in phase, 90 degrees behind the phase of the voltage, thus producing a magnetic field out of phase with the magnetic field produced by the coarse wire coil. These two magnetic fields act on an aluminum disc armature D and cause it to rotate like the armature of an induction motor. Permanent magnets are associated with
the disc so as to cause the retardation of the rotation as in the Thomson watthour meter.
Sometimes a continuous record of amperes, volts or watts is desired and in that case the pointer of an indicating instrument is tipped with a pen which plays over a moving dial or strip of paper, thus leaving a record of changes in the circuit. Instruments thus equipped are called curve-drawing instruments. Fig. 13 shows part of a record taken from a curve drawing wattmeter.
Various other instruments are used in elec tric circuits, such as those indicating the power factor of alternating current circuits, called power factor meters; those indicating the fre quency of alternating currents, called fre quency meters; and those indicating the phase and frequency relations between the currents in two alternating current circuits, called phase meters or synchroscopes. Descriptions of all such special instruments may be found in standard works on electrical engineering.
For further information in regard to electrical measuring instruments con sult Jackson, D. C. and J. P.,