222 Mr. F. B. Pidduck on a Method of Measuring 



these are such that the oscillations of the ring can rarely be 

 reduced below 5 mm. on the scale. To minimize th^m it 

 was decided to keep the current constantly adjusted with a 

 Kelvin's ampere balance as indicator, and to use a strobo- 

 scope indicator for the speed. The axle of the alternator 

 carried a brass disk 10 cm. in diameter, pierced with two 

 slits 3 cm. long by 2 mm. wide, equidistant from the centre 

 on opposite sides. On one side of the disk was a small glow- 

 lamp, on the other an electrically maintained tuning-fork of 

 frequency 50. The intermittent illumination makes the fork 

 appear bent into a sinuous form (with a curious appearance 

 of melting), the waves moving backwards or forwards along 

 the prongs with a frequency 2^—50. The sinuosities are 

 kept steady with a field rheostat on the D.C. side, and thus 

 g is kept at 25, within less than 1 per cent. 



The alternating magnetic field was produced with a pair 

 of coils arranged in Helmholtz's fashion for a uniform field. 

 Each coil had 500 turns, and the average of their mean radii, 

 was 17*82 cm. When placed with their centres at 17*72 cm. 

 apart they gave a field of 25*4 gausses per ampere. The 

 suspended circuits were three in number : — 



(1) A copper ring of mean radius 2*030 cm. and diameter 



of wire 199 cm , mass 3*666 gm. The calculated 

 moment of inertia about a diameter was 7*560 C.G.S. 

 units, its mean area 12*95 cm. 2 



(2) An aluminium ring of mean radius 1*735 cm. and 



rectangular section, axial width 0*33 cm. and radial, 

 width 0*35 cm., mass 3"085 gm. The calculated 

 moment of inertia was 4*685, its mean area 



9*460 en 



(3) A solenoid of 54 turns of enamelled S.W.G. 20 wire r 

 closely w^ound on a wooden core. Mean diameter 

 10*05 cm.,, length 5*45 cm., mass 267 5 gm. Its 

 moment of inertia, measured with a bifilar, was 

 3864 C.G.S. units, its mean area 79 4 cm. 2 



There was also an added moment of inertia of from 0*005 

 to 0*023, according to the size and position of the attached 

 mirror. The alternating current in the coils was 0*5 ampere 

 (effective value). The suspension w T as chosen to give a time 

 of swing of the order of 20 seconds, a thin galvanometer 

 suspension for the two rings, and a copper wire for the 

 solenoid. Damping took place under a steady current of 

 1 to 2 amperes, which increased the mean natural X by a 

 quantity of the order 02. 



The following are the results of four experiments on each 



