42 HEPORT — 1876. 



mittee on Electrical Standards. To m is soldered one of the battery terminals. 

 The connexions on the right are similar to those on the left, and may be un- 

 derstood from the diagram. The other galvanometer terminal goes to the 

 contact-block L. The battery used consisted of twelve Leclanche's cells, the 

 Avhole internal resistance of which was about 13 B.A. units, its E.M.F. being 

 about 16 times that of a Dauiell. The whole resistance of the bridge from F 

 to Gr was about -075. The galvanometer is an instrument made by EUiott 

 Brothers, belonging to the British Association ; its resistance is about half a 

 B.A. unit. 



Good contact between the feet of the copper terminals of the quintuple coil 

 and the bottom of the mercury-cups was secured by placing a weight on the 

 top of the coil ; the spring in the terminals was thou sufficient to ensure 

 contact everywhere. 



In the arrangement figured in the diagram the coil p is balanced against a 

 multiple arc, containing q and r in one branch, and s and t in the other. 

 To compare one single coil with the next single coil, I is removed, and one 

 end of the galvanometer wire connected instead with the cup E, while m is 

 made to connect the lower instead of the upper half of A with u ; with this 

 arrangement the coil t is balanced against the coil s. 



The coils in the quintuple coil are numbered 1, 2, 3, 4, 5 ; and in experi- 

 ments with multiple arc the coil between A and B is referred to as the 

 " single coil ; " in experiments with single coUs those between D and E 

 and E and A are called right coil (E.G.) and left coil (E.G.); the coils 

 between iv and .v and u and v are called right and left middle coils (E..M.C. 

 and L.M.C.), and are numbered 1 and 2. The bridge is read from left to 

 right. 



Some preliminary experiments were made with the apparatus, which 

 showed that the coils had been very well adjusted by the makers, Messrs. 

 Warden, Muirhead, and Glark. It was found that with the arrangement 

 described (the best at our command in the Cavendish Laboratory), the bridge 

 could be read to a quarter, if not to an eighth of a millimetre. A small 

 correction was found necessary for the magnetic field, due to the current in 

 the bridge connexions ; this was allowed for by adjusting a loop of the 

 battery-wire till the galvanometer showed no effect when the battery was 

 turned on. Thermoelectric currents in the galvanometer circuit, owing to 

 heating from the hand at the contact-block, were avoided almost entirely by 

 using two pieces of wood, which were interposed between the fingers and the 

 block, and were continually changed so as not to get hot. 



The order of experiment was generally as follows : — The weight was ad- 

 justed on the quintuple coil, the battery was thrown in for a moment by 

 means of a treadle which closed the battery circuit ; if there was no direct 

 effect on the galvanometer, the battery was thrown out, and contact made at 

 the block ; the spot of light on the scale was watched through a reading- 

 telescope, and if it was at rest* the battery was thrown in : the deviation 

 indicated which way the block had to be moved to get a balance. Two or 

 three trials in general sufficed to get the balance. Tlic bridge was then 

 read ; the middle coils were then reversed, the balance foimd, and the bridge 

 read again. The difference of the readings gives the difference of the resist- 

 ances of the middle coils, as may easily be shown (see ' Journal of Society of 

 Telegraph Engineers,' Oct. 1872). The middle coils being replaced as before, 

 the quintuple coil was moved round one step, and the same process repeated. 



* On the avoidance of small thermoelectric effects, see below iu the discussion of the 

 second experiment. 



