56 



JIEPORT 1876. 



rig. 7. 



v@^, 



The action of PQ, is obvious. "VMjen the prongs approach each other the 

 iipper dipper is depressed into the mercTiry iu S, while tlie lower dipper is 

 raised out of the mercury in T, so that the current of the larger battery 

 passes, aud vice versd when tlio prongs separate ; and it is easy enough liy 

 throwing a galvanometer iu instead of one of the batteries, and then setting 

 the fork going with the other on, to adjust the break in such a way tliat there 

 is perfect indepojidenee between the two currents. This test was in fact ac- 

 tually applied either at the beginning or end of each set of experiments. Wo 

 have thus alternately sent through the bridge certain definite fractions of 

 the whole current due to the large and small batteries. What fractions these 

 are will depend on the nicety with which the break is adjusted (with perfect 

 adjustment it would be one half of each), and also on the state of the mercury 

 surfaces and of the dippers. As may be imagined, the main difficulty of the 

 experiment lay in getting the dippers to work properly. Several sorts were 

 tried; plain copper amalgamated was found to act fairly well, but broad 

 spade-shaped pieces of platinum-foil answered on the whole best. The sur- 

 face of the mercury was covered with spirit, which is effectual so far in x>re- 

 ventingthe spoiling of the surface ; but ultimately the cups get clogged with 

 finely divided mercury, and tlien all regular action is at an end. It was 



