342 J. Troiobridge — A Telephone Relay. 



field by the effect of feeble currents circulating around coils 

 placed in such fields. Suppose, for instance, that we have a 

 hollow electromagnet with another electromagnet suspended 

 above it, the iron core of the suspended magnet forming a part 

 of the core of the stationary more powerful electromagnet. 

 The system can evidently be balanced, in various ways ; for 

 instance, the suspended magnetic core can be maintained in a 

 definite position by connection with a telephone diaphragm ; 

 and when- a feeble current circulates through the coil of such 

 a suspended electromagnet its position with respect to the 

 stationary coil is changed. Instead of the diaphragm of a tele- 

 phone, it is evident that a diaphragm connected to a micro- 

 phonic contact can be employed. This idea can be found in 

 the efforts of many inventors to construct a relay. Professor 

 Dolbear has described a telephone which works upon this 

 principle ; a non-magnetic diaphragm placed close to the pole 

 of a permanent magnet carries a little electromagnet which is 

 balanced under the influence of the elasticity of the diaphragm 

 and the magnetism of the permanent magnet. When the 

 voice causes the diaphragm to vibrate, the movements of the 

 little electromagnet disturb the magnetic field, producing 

 feeble currents of induction in the little moving coil which 

 transmit speech to a similar piece of apparatus. If this similar 

 receiving apparatus of Professor Dolbear had been employed 

 to modify a microphonic contact, it would have been the pre* 

 cursor of many subsequent inventions. 



Instead, therefore, of the turning or torsional effect relied 

 upon to actuate Lord Kelvin's siphon recorder, — called 

 " Siphon " because a siphonic pen records the oscillations of 

 the vibrating coil, — we have efforts to utilize the to and fro 

 thrust of a vibrating core of an electromagnet whose position 

 in a powerful field is modified by the strength of the feeble 

 telephonic currents which circulate around the core of such a 

 magnet. 



At first sight it would seem that the inertia of the suspended 

 electromagnet, or that of its core or plunger if the coil of the 

 electromagnet is fixed, would be so great that the motion of 

 the microphonic contacts would be seriously impeded. It is 

 true that the weight of the vibrating parts in this form of 

 relay must be small, and there must not be any subsidiary 

 vibrations of the moving parts which might be superposed 

 upon the vibrations due to the telephonic currents. It is evi- 

 dent that such subsidiary vibrations can arise if the moving 

 parts are long and of considerable size. With a loaded micro- 

 phonic contact we can have feeble effects or roaring sounds, as 

 if the membrane of the ear is loaded by an obstruction. 



