410 Mr. T. Gray on the Electrolysis 



of his "school galvanometers'''' arranged so as to have a 

 " false zero/' and having its needle suspended in a strong 

 field produced by permanent magnets, was used. An ordinary 

 index galvanometer when arranged in this way, with its zero well 

 off the scale, gives ample sensibility, and is very compact and 

 convenient. The current was adjusted by the rheostat until 

 the index was exactly over a division of the scale, and it was 

 kept there by turning the rheostat, if the current varied, all 

 through the experiment. 



Standardizing Arrangements. — The arrangement of the 

 apparatus for standardizing instruments intended to measure 

 strong currents such as deca and hecto ampere-metres is shown 

 in figure 7, which may be taken as a plan of the arrangements 

 on the top of the standardizing table, with the exception that 

 the galvanometer G is, for convenience in the figure, shown 

 much too close to the conductors. On one end of the table 

 six of the Electric Power Storage Company's accumulators are 

 arranged, and connected by means of thick copper rods to a 

 mercury-cup commutator as shown. The mercury cups m are 

 shown joined across by bridges which are of thick copper in such 

 a way that the cells are in series, but for most purposes these are 

 removed and the cells joined in parallel by means of two rods of 

 copper, provided with teeth at the proper distances apart to fit 

 into the cups m, and thus join all in each row together. The 

 battery, when fully charged and joined in this way, is capable of 

 maintaining a current of two hundred amperes for ten hours. 

 The commutator is joined by means of two copper rods r, r, 

 to a distributing board A, by means of which one or more 

 instruments can be put in the same circuit. In the arrange- 

 ment shown, a set of conductivity-bridges D and a rheostat 

 R are introduced between the cups 1 and 2 ; a galvanometer 

 G between the cups 4 and 5 ; a pair of large electrolytic cells, 

 joined together by means of a moveable cup M, between the 

 cups 6 and 7 ; and an electric-current balance between 11 and 

 12. The arrangement here shown, together with details of 

 the conductivities D and the rheostat R, will be found 

 described by Sir William Thomson in a recent patent specifi- 

 cation. The following description will give a general idea of 

 the apparatus and the mode of using it. 



A perspective sketch of two of the conductivity-bridges D 

 is given in figure 8, where one that is in the circuit is shown 

 standing in the mercury troughs t, t, and another insulated. 

 These conductivity-bridges consist of U-shaped pieces of plati- 

 noid rod or wire according to the conductivity required. The 

 thick rods are bent into shape and the two limbs held at the 

 proper distance apart by wooden blocks, while the thin wires are 



