ON OHM S LAW. 



41 



cut as nearly as possible of the same length, so that their amalgamated ends 

 might go in pairs into mercury-cups. The wire and bobbin were enclosed 

 between two coaxial cylinders of sheet brass, which were fastened to the 

 ebonite piece above, and connected by a ring of sheet brass beloAV. The 

 whole had a rough resemblance to a large spider. The other set consisted of 

 two coils made of the same wire, and having each as nearly as x^ossible the 

 same resistance. They were arranged in the same way, except that the ter- 

 minals of the same coil were adjacent. 



As the adjustment of the coils was necessarily not perfect, the experiment 

 could not bo tried exactly as described in the above scheme. I decided, 

 therefore, to operate as follows : — First, to compare each coil of the five with 

 the coil next in order ; the differences between any two coils could then be 

 found in terms of an arbitrary unit (the resistance of a tenth of a millimetre 

 of the platinum-iridium bridge wire at the temperature of the room during 

 the experiment) ; second, to compare each coil with the f oux others arranged 

 in multiple arc, as before described. The results thus obtained were com- 

 pared, as will be described further on. 



To facilitate these comparisons, the following aiTangement of mercury-cup 

 connexions was made for me by Mr. Garnctt, of fit. John's College, the 

 Demonstrator at the Cavendish Laboratory : — 



Fig. 3. 





To a massive board are glued five large mercury-cups, made of boxwood, 

 with a piece of amalgamated sheet copper at the bottom. Into these go the 

 ten teiininnls of the five coils, so that there would be metallic connexion 

 round all the five coils in scries were it not that the cuj) A is divided by a 

 piece of vulcanite, which insulates the two terminals in that cup. ? is a 

 stout copper bow connecting B and the lower division of A ; to this bow is 

 soldered one of the galvanometer terminals. Into the cups u and v dip the 

 two terminals of one of the two coils, m is a stout bow of copper connecting 

 the upper half of A with u. Another bow goes from v to F, one end of the 

 bridge, which is the instrument used by the British-Association Committee of 

 1863, and will be found described at p. 353 of the Eeport (1864) of the Com- 



