Rayleigh's Alternate Current Phasemeter
has described an instrument for measuring the angle of lag of an alternating current behind the E.M.F. impressed at the terminals of the circuit. With proper calibration the readings give also the current, E.M.F., and thus generally such quantities as apparent watts, true watts, power-factor, coefficient of self induction, PR loss, hysteresis, etc. The work described in the following pages was undertaken with the hope of obtaining an instrument suitable for the measurement of the quantities
... the quantities above mentioned by students in the laboratory. It has been shown by Breslauer 2 that the instrument is particularly well adapted for measurements of activity in circuits of low power-factor such as motors running light and choking-coils with open magnetic circuits. He has also discussed the advantages that it has over the electrodynamometer used as a wattmeter on circuits with low power-factors. Description and Theory of the Instrument.-The instrument consists of a current coil, M t and an E.M.F. coil, S, arranged as shown in Fig. 1 (p. 281) . A piece of soft-iron wire, m, I cm. long, is suspended by means of a phosphor-bronze torsion wire, at an angle of 45 0 to the common axis of the two coils. Each coil is made adjustable along this axis. The deflection of the needle is measured by that of a ray of light reflected from a concave mirror attached to the suspended system. The couple acting on the needle due to the current in either coil acting alone varies as the square of the current and as sin 2d, where 0 is the angle between the needle and the axis of the coils. The couple thus becomes a maximum for d = 45 °, and zero for 0 = o and 0 -90.