222 



Mr. F. W. Burstall on the Use of 



on the bridge-wire. This gave a value of '00967 ohm at 

 12 0, 8. Secondly, the first coil of the ^ ohm dial, together 

 with the leads, was balanced against the t X q ohm standard ; 

 subtracting from this resistance the known resistance of the 

 box-coil, we get a value of '00964 ohm for the resistance of 

 the leads, at 12°'4. Thirdly, a similar method was employed 

 to measure the first coil in the 1 ohm dial, and this determi- 

 nation gave the resistance of the leads as '00954 ohm at 

 10°' 3. In order that these three observations may agree it 

 is necessary to assume that the temperature-coefficient of the 

 copper leads is about '00005 ohm per degree. This is rather 

 high, but the temperature range is too small for an accurate 

 determination. 



The values of the coils in the 100 dial were obtained on 

 the box itself. In the gap A (fig. 3) the 1 ohm standard coil 



Fig. 3. 



I OHM 

 STANDARD 



WW1 



was placed ; this, together with the first three dials, gave a 

 resistance of about 100*9 ohms ; and constituted one arm of a 

 Wheatstone bridge, the two 10 ohm coils in the proportional 

 arms formed two other arms, the fourth arm was any one of 

 the 100 ohm coils, which was connected to the gap C by 

 means of the two flexible leads with conical ends. The plug 

 from the centre of the 100 dial was in the zero of the 1000 

 dial so as to complete the circuit from the gap A to the gap 

 C. The smallest resistance in the box was -^ of an ohm. so 

 that the last two figures of the resistance had to be obtained 

 by interpolation from the swings of the galvanometer. The 

 battery employed was one storage-cell, and the current was 

 commuted for each observation. With the coils of the gal- 

 vanometer placed in series an alteration of -^ of an ohm in 

 the box caused a total galvanometer swing of about 116 scale- 

 divisions. 



