Driving Tuning-Forks Electrically, 491 



most unfavourable moment, i. e. when it is stationary. The 

 impulse can be considerably delayed by including in the battery 

 circuit a solenoid into which a solid iron core is inserted 

 more or less till the best result is obtained. This adjustment, 

 however, varies with the amplitude of vibration and the al- 

 terations of the contact. The whole matter was discussed in 

 a paper read before the Physical Society on June 26th, 1886, 

 by Prof. S. P. Thompson*, who then suggested the employment 

 of two similar forks which drove one another and settled down 

 to a difference of phase of a quarter of a period. Each fork, 

 therefore, received an impulse when moving at its greatest 

 velocity in the middle of its stroke. 



The method I now suggest accomplishes the same thing 

 without the employment of a second, fork, and moreover gives 

 two impulses in each complete period instead of one. 



The arrangement is shown in fig. 1. 



Pig. 1. 



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F is a tuning-fork mounted in the usual way, and provided 

 with a driving electromagnet M, and contact-maker K. But 

 instead of taking the main battery-current directly through 

 the electromagnet M, it is sent through the primary coil of 

 a kind of transformer T, the secondary circuit of which is 

 connected to the electromagnet. Hence, at both make and 

 break of the main circuit, momentary induced currents pass 

 through the driving magnet alternately in opposite directions. 

 By polarizing the fork so as to make it a horseshoe-magnet, 

 the impulses are also made to act alternately as attractions and 

 repulsions, and the fork receives two impulses for each com- 

 plete vibration. By properly adjusting the contact when the 

 fork is at rest, so as just to complete the circuit, the impulses 

 will be given very approximately at the middle of the stroke, 

 and are independent of the amplitude of vibration. The fork 

 I tried made about 86 vibrations per second, and had prongs 20 

 cms. long and 1*8 cm. apart. The electromagnet M was formed 

 by winding about 50 turns of no. 22 silk-covered copper wire 

 round a core formed of a bundle of varnished iron wire. It was 

 mounted on a wooden support capable of sliding between and 

 parallel to the prongs. The transformer T consists of a core 

 * Proc. Phys. Soc. viii. pt. ii. ; Phil. Mag. [5] xxii. p. 216. 



