288 



Mr. P. E. Shaw on some 



soft iron is seen to be ABCDA ; it has the air-gap betwv. 

 D and A in which the coil n n lies. This is the coil M^ or M 2 

 in fig. 2. 



The coil n n is very light, 60 turns of covered copper wire 

 (36 S. W.G.), resistance about 4 ohms, wound on a paper pill-box, 

 which is light and rigid and perfectly circular, and is hence a 

 very convenient former for the coil. The steel wire s s has a 

 light brass fitting screwed tightly to its centre ; below this is 

 a light bow of cedar lashed and cemented to the brass above 

 and lashed and cemented to the coil below. 



As seen in the sketch, the wire is fixed (bent on itself and 

 brazed) to a brass screw at each end, each provided with two 

 nuts, on the outside of the stout brass frame b b. 



By tightening the wire we can produce in it the required fre- 

 quency, 400 (this can be observed by gently tapping the wire 

 and listening for its natural frequency) ; then alternations in 

 the current of n n must produce vibration in the wire s s, and 

 when the carbons are set by the screw z we have the micro- 

 phone acting. 



Two graphite buttons, or one graphite and one hard carbon 

 button, were found to act very well. 



Another form of relay was used. Take a Blake transmitter, 

 change the stiff iron plate for a more yielding one, and attach 

 the coil n n to it. This acts fairly, but is obviously not 

 syntonic with the whole system as in the other case. 



Manipulation. 

 The chief special difficulties may be mentioned categorically. 



I. It must be borne in mind that in such an apparatus as 

 ■p 



that used, where the ratio j- is comparatively large, damping 



occurs rapidly, and consequently the current-amplitude dies 

 out rapidly between each fresh charge received from the fork. 



Fig. 4. 



We supposed the fork to have frequency 100, and the circuit 



to have frequency 400. Then the curve for current would 



be of the nature shown in I.; whereas the ideal curve, where 

 ■p 



-j- is negligible, would be as in 11. } where the amplitude is 



