Lecture Experiments illustrating Syntony. 289 



seen to remain constant. Consequently in our system we 

 should have a pronounced fundamental 100 as well as an 

 harmonic 400, and we have to learn to pick out the latter and, 

 in adjusting the circuit, to work it up to a maximum. This 

 applies to both primary and secondary circuits. 



JI. Care must be taken to properly adjust the platinum 

 and mercury contacts, not only to produce the maximum true 

 sound in the circuit, but also to avoid any spurting at the 

 contact ; this would have its counterpart in violent spurts in 

 the sounds in the coils, which mask the true effect. 



III. The greatest difficulty experienced, however, was in 

 the microphones, both (1) in obtaining them truly syntonic 

 with the circuit and with one another, and (2) in the minute 

 adjustment of the carbons for very weak currents. The 

 special relay described above worked well, but requires 

 delicate handling in respect of both (1) and (2). Anyone who 

 has worked a granular transmitter, or indeed any microphone 

 with a view to obtaining maximum effects for very small 

 movements, will admit that an exceedingly fine adjustment is 

 required. This might seem to be self-evident, but the author 

 has made independent research on this point, his results being 

 that a movement o£ one carbon of the order 10 ~ 5 cm. would 

 carry the microphone from hard contact (no microphone 

 action) to complete freedom. Hence the difficulty above 

 stated is quite intelligible. 



In working the relay described previously, the final ad- 

 justment was made by gently pressing with the finger on the 

 stout brass bar bb (the section of the bar is \ in. X \ in.). 

 This of itself indicates the delicacy of the microphone. 



IV. The design of the sending and receiving coils. 

 The author had in each of them 100 turns of insulated copper 

 wire (16 s.w.G.), radius about half a metre, resistance about 

 3 ohms. If the same wire had been opened out to 50 turns of 

 radius one metre in both sender and receiver, the mutual 

 induction would be four times as great ; the self-induction of 

 each would be slightly reduced (roughly to about two -thirds 

 of its former value), but this could be made up in the auxil- 

 iary coils. Thus it is advantageous to have the radius o£ the 

 coils large. In making the auxiliary coils we require to 

 have large L with small R. If we have a given amount of 

 wire at our disposal, it is better to have small radius and many 

 turns, and have a core of iron wires for final adjustment. 

 One advantage of keeping It moderately large in the circuit 

 is that we run no risk of breaking down the condenser in the 

 primary. 



V. In Prof. Lodge's research the sound produced at the 



