24 Messrs. Hutchinson and Wilkes's Comparison of 



Resistances. — The resistances used were : — 



1. Warden-Muirhead 10 B.A.U., No. 292. Value, de- 

 termined by Glazebrook, October 1887, 9*99416 at 16°'5 0. 

 Temperature-coefficient "00292. This coil was our standard. 



2. Elliott 10 B.A.U., which has been several times com- 

 pared at the Cavendish Laboratory. It is marked as found 

 correct by Rayleigh at 20 o, 9. By comparison with W. M. 

 No. 292, we found it correct at 20°*8. Temperature-coefficient 

 •0034. 



3. Elliott 1 B.A.U. Resistance -99950 at 16° C. Tem- 

 perature-coefficient *00037. 



4. Pratt 1 B.A.U. Resistance 1-02579 at 16° C. Tem- 

 perature-coefficient -00030. 



5. A circular comparator (designed by Professor Rowland), 

 containing ten 10 B.A.U. coils wound together on a copper 

 cylinder, which contained water. The coils are protected by 

 an outer cylinder which leaves a large air-space. Contacts 

 are made by means of mercury cups, arranged circularly 

 in an ebonite top. This comparator was used in standardizing 

 the one-ohm coils and as a shunt. The coils were always kept 

 standing in water for several hours before being used, and the 

 temperature of this water was kept perfectly constant through- 

 out the day by having in each vessel a coiled lead pipe, which 

 was connected by rubber tubing with the pipes of the city 

 water-supply. 



All resistances were compared both before starting and 

 after completing this portion of the work, and the two sets of 

 results agreed. 



The resistance of the rods used to connect the end terminals 

 to the bridge was calculated for temperature of room, 18° C, 

 •001257. By observation : — temperature of the room 21°*5, 

 •001258. As this temperature never varied much from 2*2°, 

 •00127 was used as the correction in all cases. 



A Fleming bridge was used, and the value of a division of 

 the bridge-wire by two distinct determinations gave agreeing 

 results. The resistances were so combined (except in the case 

 of the first measurements made) as to require the use of as 

 small a portion of the wire as possible. 



Tube I. was balanced against coils E 1 B.A.U. and P 1 B.A.u. 

 in parallel, shunted by coils 1, 2, 3, 5 of comparator in series. 

 In 1st observation, shunt = coils 1 + 2. 



Tube II., against E 1 B.A.u. + P 1 b.a.u. in series, shunted by 

 coils 2, 3, 4, 5, 6 in series. In 1st observation, shunt = coil 1. 



Tube III., broken in preliminary work. 



Tube IV., against W.M. 10 b.a.u. 



