444 ENGINEERING ON THE FARM 



direct-current receivers. In these the permanent magnets 

 have been eliminated and the necessary magnetism is derived 

 from the batteries through the effect of the current on the 

 winding of the receiver magnets. Since a transmitter is 

 essentially an instrument for varying the resistance of a 

 circuit, its effectiveness is dependent upon the extent to which 

 it can vary the resistance in that circuit. For example, a 

 circuit containing two transmitters, two receivers, a battery, 

 and half a mile of wire will measure approximately 200 ohms. 

 The internal resistance of a transmitter in operation will 

 vary from 5 to 50 ohms, a net variation of 45 ohms. This 

 is 22^ per cent of the total resistance of the circuit. 

 Now suppose the length of line is increased until the total 

 resistance of the circuit reaches 1,000 ohms. Either trans- 

 mitter will be able to vary the resistance of such a line only 

 4}^ per cent. Since the volume of sound reproduced by the 

 receiver is dependent upon the changes occurring in the 

 current flowing through its winding, it will be apparent that, 

 even though the voltage of the battery is such as to cause the 

 same amperage to flow in the circuit in both cases, the 

 volume of sound which can be produced under the second 

 condition assumed would be materially smaller than it would 

 be with the shorter line. 



Other considerations which have a bearing on the fore- 

 going and which will assist one in grasping the limitations 



of the series circuit are: (1) 

 It is not possible to increase 

 the volume of transmission 

 by increasing the current 

 through the transmitter, 

 thus compensating for in- 



Q Ringer 



[|j Condenser ^ 



1 



Fig. 337. Circuit for common battery . * ' . .. 



series telephone used on lines up Creases in lengths OI lines. 



to two miles ^ The parts of ^ i nstm _ 



ments as at present designed and constructed for resistance 

 variations such as those cited will withstand only a rather 

 definitely limited amount of current without overheating 



