196 Proceedings of Indiana Academy of Science 



If the resistance Ri is set at some convenient value, 10 ohms, say, 

 and R: is adjusted to minimum sound, then the ratio of R2 to Ri is the 

 amplification constant of the amplifier. 



A=R2/Ri= Change of plate potential divided by the change of grid 

 potential. If the amplification constant of the tubes has been deter- 

 mined then the amplification constant of the audio coil can be deter- 

 mined. A=aiCia;C2a3 — a,,. 



Where ai, a2, — a „ is the amplification constant of the various tubes, 

 and Ci, C2 — is the amplification constant of first, second, etc., coils. 



Best results are obtained with two tubes and one coil. The plate 

 potential must be such as to let the tubes operate on a point near the 

 middle of the plate current, grid potential curve. C batteries of suita- 

 ble value may be inserted in the grid circuits of the tubes, if necessary. 



In a resistance amplifier (fig. 1, resistance amplifier) an uneven 

 number of tubes must be used. If the grid of the first tube is increased 

 the plate current increases and the potential of the plate diminishes, 

 causing the potential of the grid of the second tube to diminsh and the 

 plate current of the second tube to diminish. This will cause the poten- 

 tial of the second plate to increase and the grid of the third tube will 

 increase and the current in the plate circuit of tube three will tend to 

 increase. 



The i-esistances in the potentiometer can be adjusted until there is 

 minimum sound as in the first case. A mil ammeter can be inserted in 

 place of the telephone and a battery of a few volts can be placed around 

 a c. A key, K, in the battery circuit is opened and closed and the 

 resistances adjusted until there is no change of deflection. 



In this resistance amplifier the actual amplification will be less than 

 the product of the amplification constants of the three tubes. If the 

 resistance, r, is four times the resistance of the tube, then the am- 

 plification is four-fifths that of the tube alone. The potential of the B 

 battery must be large. If the tube is to work with 40 volts potential 

 on the plate then the battery must be 200 volts if r is four times the 

 I'esistance of the tube. 



