16 RFPORT 1876. 



louad impossible to avoid tliis disturbance altogether ; and accordingly the 

 following mode of procedure was adopted : — 



Direct magnetic disturbances. — We first carefully investigated whether 

 there was any direct magnetic effect on the galvanometer owing to the 

 currents in the apparatus ; this was done by simply short-circuiting the 

 galvanometer. No such effect could be detected. Being assured of this, we 

 always operated as follows : — Threw in the galvanometer by pressing down the 

 button, then allowed the needle to come to rest with the small permanent 

 deflection due to the thermoelectric current. If now, on pressing down the 

 treadle for an instant, there was no motion of the spot, we concluded that 

 there was a balance. It is to be noticed that since we are near balance 

 the battery-circuit is conjugate to the galvanometer- circuit, and that, there- 

 fore, making or breaking the battery-circuit does not alter the effective resis- 

 tance opposed to any electromotive force, thermoelectric or other, in the 

 galvanometer-circuit. (Of this we also assured ourselves by direct experi- 

 ment.) Another advantage of this method is that it ensures the least 

 possible use of the battery, and thus avoids disturbances from heating. 

 During our final experiments both of us had acquired by considerable j^rac- 

 tice an acquaintance with the indications of the galvanometer, which enabled 

 us to adjuib^t the balance quickly, and thus secure in greater measure the 

 advantage above mentioned. 



Self- and mutual induction. — It is also worth remarking that from the 

 way the B.A. unit coils are wound, and from the general arrangement of 

 the apparatus, neither self- nor mutual induction could have any sensible 

 disturbing effects in our experiments *. 



Method of using bridge for finding coejjUcients of variation Sfc. — In finding the 

 variation-coefficients of the coils the bridge arrangement was used in the way 

 described in the Report on Electrical Standards, 1864 (p. 353, &c.) ; but in 

 finding the difference between the resistances of two coils, the method de- 

 scribed by Prof. Foster (in the Journal of the Society of Telegraph Engineers, 

 October 1874) was used. In this method the bridge is first read with the 

 normal coil and the coil to be compared- with it in one position, and then 

 the coils are interchanged ; the difference of the bridge-readings gives the 

 required difference of resistance in units of the bridge. 



Bridge-miits. — The unit in which we shall state our results further on is 

 the resistance of a tenth of a millimetre of the bridge-wire, which is a metre 

 long and has a resistance of about •075 ohm. 



Calibration of bridge and thermometer. — The wire was carefully calibrated, 

 but no errors were found large enough to affect our results. 



The thermometer used was also compared with a standard thermometer 

 belonging to the laboratory, and the corrected temperatures are in every 

 case given. The degree of accuracy attained in this last compaiison was pro- 

 bably about -05 Centigrade. 



Description of coils in the case. — In the case containing the coils there 

 are altogether fourteen coils. Five of these are multiples of the unit, viz. 

 2, 3, 5, 8, and 10, and have brass labels on them ; but the inscriptions have 

 never been completed by filling in the last two figures of the temperature at 

 which they are equal to the standard. We have not been able to get any 

 description of these whatever, and have therefore not measured them. 

 Besides these there accompany the box two coils marked A and B, which 

 are not units, and a flat coil described as a normal coil, besides a set of 



* Another precaution of less importance was to cover the platinum-iridium wire of the 

 bridge witii pieces of wood to screen it from dust and radiation from the body of the 

 observer. 



