332 



.Lord Rayleigh : Acoustical Notes. 



shaded parts 

 Fig. 6. 



accompanying diagram (fig. 6) where the 

 represent brass pieces separated by ebonite 

 insulation. Springs lightly bearing against 

 the exterior continuous portions o£ metal 

 correspond to B and C, while A corresponds 

 to a brush bearing near the centre and 

 making contact alternately with the two 

 metal pieces. Provision was made for varying 

 the pressures at these contacts during the 

 running and without disturbing the insulation. 

 The problem is to secure a uniform rotation of 

 this commutator, whose diameter was 28 mm. 



The phonic wheel, mounted on the same 

 shaft as the commutator, takes its time from 

 a vibrating fork (44 per second *) acting as in- 

 terrupter of an electric current. The current 

 (about 4 amperes) is from 3 secondary cells and excites not 

 only the electro-magnet by which the vibrations of the fork 

 are maintained but also the electro-magnet of the phonic 

 wheel. Four soft iron armatures are mounted round the 

 circumference of the drum and in their passage complete 

 approximately the magnetic circuit. The holes through 

 which the fork is viewed are also four in number. 



The most advantageous action of the regulating current 

 occurs when one armature passes for each complete vibration 

 of the fork. Under these circumstances the prong, or 

 rather a projecting wire attached for the purpose, is seen 

 stationary and single. There are then 11 ( = ^x44) revo- 

 lutions of the wheel per second and 22 charges and dis- 

 charges of the condenser. But the wheel may also be run 

 at double or triple this speed, and then the projecting 

 wire is in general seen doubled or tripled. The regulating 

 current from the fork is of itself capable of maintaining 

 the rotation at single or double speed when once the 

 necessary engagement has been secured. For this purpose 

 the speed must be raised to the required point by means 

 of a string passed round the shaft and worked with the 

 fingers, and even then it may be only after many trials 

 that engagement ensues. For the triple speed the power of 



* For the design of steel vibrators and for rough determinations of 

 frequency, especially when below the limit of hearing, the theoretical 

 formula is often convenient. We may take 



frequency = 84600£/Z 2 , 



Avhere I is the total length of a prong and t the thickness in the plane of 

 vibration, both being reckoned in centimetres. (' Theory of Sound,' § 177.) 

 At any rate the octave is never uncertain. 



