Maintaining Tuning-forks by Electricity. 217 



tion, the difference arising from the elasticity of the stylus, 

 and the necessary imperfection of the contact until a certain 

 actual contact-pressure has been attained. There is also a 

 certain retardation in the electro-magnetic pull behind the 

 instant of greatest current, owing to the self-induction of 

 the circuit and the mutual induction between the coil and its 

 core. But if the core be short, and laminated, and of good 

 iron, and if the resistance of the circuit be considerable in 

 proportion to its coefficient of self-induction, the retardation 

 of phase in the periodic electromagnetic impulses will not be 

 of any great importance. Lord Ray lei gh remarks* that if 

 the magnetic force depended only on the position of the fork 

 the phase might be considered to be 180° in advance of that 

 of the fork's own vibration. That is to say, considering a 

 displacement of the fork toward the electromagnet positive, 

 the maximum force occurs when the displacement is a nega- 

 tive maximum. But, adds Lord Rayleigh, the retardation due 

 to self-induction and imperfect contact reduces this advance. 

 Lord Rayleigh further remarks that if the phase-difference be 

 reduced to 90° the force acts in the most favourable manner, 

 and the greatest possible vibration is produced. He might 

 have added that in this case the tendency to produce phase- 

 change is the least possible. He suggested as a means of 

 producing any desired retardation, the use of a stylus attached 

 not to the prong itself, but to the further end of a light 

 straight spring carried by the fork. 



It seems to the present writer, that a better way to secure 

 the proper timing of the impulses is to be found in the sug- 

 gestion which he now makes, and which arose in his mind 

 after considering the difference of phase which exists between 

 two dynamo-electric machines associated together. Let two 

 forks in unison with one another be provided, and let each act 

 as interrupter to the other, but not to itself; the electromag- 

 net of each being included in the circuit of the other's contact- 

 points. One battery will suffice for the two, as they will not 

 both make contact at the same time. 



Fig. 1 shows the proposed arrangement. The forks when 

 started will settle down to a difference of phase corresponding 

 to an almost exact quarter of a period. 



Fork B is arranged, as shown in fig. 1, so that it makes 

 contact at the inward stroke at the point when its displace- 

 ment is at the positive maximum ; fork A makes its contact 

 at the outward stroke when its displacement is at the negative 

 maximum. 



* Op. cit. p. 59. 



