664 



Mr. C. G. Barkla on the Velocity of 



The maximum occurred when the length of the secondary 

 circuit was about 600 centimetres ; a gradient in the curve of 

 about —"04 millimetre scale-divisions per centimetre dis- 

 placement of the bridge (which was the slope given by (5) 

 when #== half the complete length of the circuit) was given 

 with a length of circuit 8 or 10 cm. greater than that for the 

 maximum, where the gradient was zero. 



This gave for the length of the circuit about 610 cm., 

 which was the wave-length for a frequency of oscillation 

 whose wave-length along *074 cm. copper was 663*4 cm., 

 showing the velocity along *0025 cm. platinum in the circuit 

 used to be about 92 per cent, of the velocity along "074 cm. 

 copper. The damping was so enormous that the position 

 could not be determined without a possible error of 2 per cent, 

 or more ; the example is given as one in which an approxi- 

 mate correction for damping was applied. 



The greatest possible fractional change in intensity (due to 

 damping) per unit displacement of the bridge, as calculated 

 from (3) for the other wires used, would not displace the 

 maxima from true positions of resonance by an amount that 

 could be measured, any such displacement being well within 

 the limits of possible error ; so that in these cases the damping 

 effect has been entirely neglected. 



It is possible to eliminate the displacement due to damping 

 in the fine wires by making the ends of the parallel wares 

 over which KK' moves of stout copper, though the reflexions 

 at the junctions are liable to disturb the true reading. 



Such an experiment gave, when correction was made for 

 the copper in the circuit, a wave-length along the fine platinum 

 wire of about 614 cm. 

 previous result. 



The first series of experiments was made on copper wires 

 of different diameters. The primary circuit was unchanged 

 while copper wires of diameters *076, '0377, *0149, '0038 

 centimetres in succession constituted the secondary circuit. 



In each case the distance between the parallel wires and 

 the length of the bridge ¥> Y B/ was 7*8 cm. The movable 

 bridge had four turns round the detector-needle, and was of 

 approximately the same length for each of the wires. 



The correction for the length of the bridges was neglected, 

 as it was a small quantity of practically the same magnitude 

 in each case. The total length of the secondary circuit, when 

 •076 cm. wire was used and the two circuits were tuned, was 

 1298*95 cm. The position of the bridge giving maximum 

 intensity of oscillation was obtainable with great accuracy, 



which is in fair agreement with th( 



