the Self -inductance of Small Loops. 225 



Preliminary standardizations were as follows : — The mag- 

 netic field at the centre of the ring for a current of 1 ampere 

 was 6*46 gausses, by calculation. For checking, it was 

 found most accurate to compensate the field with a second 

 circuit of known dimensions, replacing R by a small magnet 

 with attached mirror. The ratio of the currents was found 

 by a bridge method while in position. Comparison was 

 made with the large Helmholtz pair, and with a square 

 frame accurately wound with a single layer of open turns. 

 These gave 6*49 and 6*45 respectively. The value 6*47 is 

 adopted. 



In order to check the constancy of current in the coils the 

 thermo-ammeter was placed (1) to the left, (2) between the 

 coils, (3) to the right, under otherwise identical conditions. 

 The mean of two observations of each current gave scale 

 readings of 54*65, 54*20, 54*45, respectively. The mean, 

 taking the middle twice, is 54*37. We therefore add 0*17, 

 or 3 parts in 1000, to the readings of the middle current. 

 This correction is near the limits of observational error, but 

 I have thought best to include it. The thermo-ammeter was 

 calibrated before and after each experiment with a Weston 

 <?ell circuit, arranged to give a current of 0*0509 ampere. 



4. Experimental 7*esults. 



The solenoid (3) was too large to use : experiments were 

 made on (1) and (2) and on the two following circuits : — 



(4) A copper ring of mean radius 0*525 cm. and diameter 

 of wire 0*054 cm., mass 0*0695 gm. The calculated 

 moment of inertia about a diameter was 0*00977, or 

 with the very small attached mirror 0*00978, its 

 mean area 0*860 cm. 2 

 (5) A solenoid of 61 turns of enamelled wire on a hollow 

 ebonite cylinder. Mean diameter 2*03 cm., length 

 of winding 187 cm. Mass of cylinder 5*802 gm.,of* 

 wire 2*082 gm. Diameter of wire over enamel 

 0*030 cm., under enamel 0*025 cm. The calculated 

 moment of inertia of the cylinder was 9*19, of the 

 wire 1*70. Mean area 3*238 cm. 2 



The deflexions, and the readings of the thermo-ammeter 

 during an experiment, were extremely steady. The scale 

 was set up at exactly 1 metre from the suspending fibre. 

 The mirror was set on accurately parallel to a large mirror 

 placed temporarily across the ring. The lamp was placed 

 quadrantally, so that a reading in the middle of the scale 



