maintaining Tuning-forks by Electricity. 219 



pulse just as B has phase 0°. A quarter of a period later B 

 makes contact (b\), and gives an impulse to A just when A's 

 phase is 0°. The arrows indicate the instant and direction of 

 the forces. It is assumed that the retardation due to self- 

 induction is small, as compared with the period of the forks : 

 and this may easily be made so, firstly, by employing only 

 short internal electromagnets with laminated cores, and, 

 secondly, by interposing sufficient inductionless resistance in 

 the circuit. 



Another point of some importance in the construction of 

 electrically sustained forks is that the contacts should be very 

 firm, and should be made much nearer the hilt of the fork 

 than is usual in these instruments. When the stylus is near 

 the outward end of the prong there is much greater amplitude 

 of motion at the contact than is really requisite. A pin of 

 platinum secured to the prong at about 5 centimetres from the 

 point of bifurcation, making contact against a platinum-faced 

 strip of German silver, half a millimetre in thickness, to serve 

 as a spring, will answer the purpose for forks of ordinary size. 



A common imperfection in the electro-diapason as usually 

 constructed is the method of mounting the fork. Its shank 

 is held with nut and washer to a block of wood or metal, 

 which is then secured to a stand by a single bolt or screw 

 which runs at right angles to the shank and to the planes of 

 vibration of the prongs. The defect of this mounting is that 

 the fork can shift a little round the bolt, and is liable to 

 . become set with one prong nearer one face of the electromag- 

 net than the other prong is to the other face. This often 

 results in the occurrence of actual rattling contacts between 

 the fork and the electromagnet, as well as in derangements of 

 the adjustment of the working contacts. It also gives rise to 

 another kind of difficulty : if one prong is nearer to the 

 electromagnet than the other is, there will be a tendency for 

 the fork to vibrate as a whole around the bolt or frame upon 

 which it is mounted, and this will give rise to slow alternations 

 of good and bad contacts, producing on the sound an effect 

 not unlike that of beats. Either the bolt which secures the 

 fork to its mounting should lie in the plane of the vibrations, 

 or else it should be replaced by a more substantial species of 

 mounting. 



