On the Measurement of Amplification Given by Triode Amplifiers at Audible and at Radio Frequencies

The measurement of amplification at audible frequencies which is described is a mutual inductance method, the alternating voltage applied between the filament and grid of the first valve being varied by quickly changing the connections to an air-core transformer having two secondaries at right angles, which secondaries are capable of rotation so as to give any desired ratio of the mutual inductances between the primary and the respective secondaries. The source of alternating voltage is a triode circuit having such capacity and inductance in the oscillatory portion as to give currents of a sonic frequency. The standard intensity of note is such as to enable signals to be read with comfort, and is obtained by adjustment of the source of alternating current. The first note is obtained when the telephone is in the anode circuit of an auxiliary valve, and the second when the telephone is transferred to the anode circuit of the last valve of the amplifying set, changes also being made in the applied alternating voltage, and in the connections between the valve circuits. All of these changes are brought about by a six-brush switch in a fraction of a second, so that readings are quickly obtained. The method is compared with a modification of the shunted telephone method, and very good agreement is obtained when the amplifying values are comparatively low. For values of a high order such as 200, it is shown that the two methods are not likely to give the same results, the method described in the Paper being the better one. In the measurement of amplification of a radio-frequency amplifier an undamped train of oscillations is broken up into groups of sonic frequency by shunting intermittently the condenser in the oscillatory circuit by an inductance of low resistance. A sonic train of oscillations is thus produced. The receiving circuit is very similar to that used for audible frequency amplification, but the measurement of the amplifying power is made by quickly varying the current at the source, and not by a change of mutual inductance, between the two circuits.