290 Some Lecture Experiments illustrating Syntony. 



receiving-telephone was very small, just sufficient for audi- 

 bility being all that is required by a single observer ; but in 

 the case here discussed, we have to produce sounds to be 

 given out to a large audience by a loud-speaking telephone ; 

 these would have to be, and were, thousands of times as great 

 as required in the former case. The process of amplifying 

 the small currents until they become large and produce 

 really loud signals is a difficult one in itself; but an even 

 greater one arises from the passage of large currents through 

 the last microphone ; the carbon-contacts are rapidly burnt 

 away, and as a microphone contact is a very delicate one, 

 continual adjustment is required. 



To meet this special difficulty Prof. Lodge has employed 

 an ingenious device which should be mentioned. The upper 

 carbon is attached to a large mass which turns freely on a 

 horizontal axis, which is so counterpoised that the two 

 carbons are gently but steadily pressed together. If the 

 carbons burn away at the contact, the pivoted system moves 

 on its axis and maintains the same microphone-action as 

 before. Thus the contact is kept automatically in working 

 order. 



It should be pointed out that though the sounds produced 

 in the receiver were much louder than in the case of Prof. 

 Lodge's system, yet the distance between sender and receiver 

 was in his case very much larger; so that each arrangement 

 has its own difficulties. 



Another Experiment. 



By a small modification in the primary circuit, tuning can 

 be demonstrated in a very striking way. In fig. 1 put a 

 loud-speaking receiver in series with the coils C x and C 2 , and 

 shunt it by a resistance-box. On working the circuit this 

 receiver gives out a loud sound of the circuit-frequency ; by 

 changes in the shunt we can affect its intensity, while by 

 changes in L or C we alter its frequency. It may be men- 

 tioned in passing that this is a convenient means of producing 

 a sound of constant intensity and is easily adjustable (though 

 not necessarily pure), and as such it has been used by the 

 author. 



As a simple example, if we change the capacity from 0'9 mid. 

 to 0-4 mfd., then the frequency should change in the ratio 



/4 2 



a/ q = q 5 the notes will be found to have about a musical 



interval of one-fifth. 



The author's best thanks are due to Prof. Lodge for advice 

 during the progress of the work. 



