the Mercury Unit of Electrical Resistance. 169 



In these measurements care was taken to fill the two tubes 

 which were to be compared from the same bottle of mercury, and 

 to keep them during the measurement at the same temperature. 

 For this purpose a zinc trough was provided, 2 yards long and 

 2\ inches broad and deep, held in a wooden box, with sawdust 

 tightly packed between the zinc and the wood. On the bottom of 

 the trough, from one end to about three-fourths of its length, were 

 cemented a series of half a dozen glass tubes, and a water-tight 

 partition was put just over their ends so as to divide the trough 

 into two unequal parts, whose only connexion with each other 

 was through the flooring of glass tubes. The trough was kept 

 three-fourths full of water, which could be made to circulate by 

 spooning it from one division into the other, and letting it run 

 back again through the glass tubes at the bottom. 



The resistance tubes were furnished at their ends with rect- 

 angular bent pieces of wide glass pipe, such as was used in filling 

 them with mercury to ascertain their capacity, held fast by india- 

 rubber stoppers. They were filled very slowly with mercury, and 

 laid into the water-trough as closely as possible, side by side. 

 The contacts between the tubes and with the measuring appa- 

 ratus were made by copper wires. The whole was protected by 

 screens of vulcanite against rays of heat from the lamps and 

 stoves of the laboratory. The temperature of the water in the 

 trough varied during a single measurement not more than 1° C, 

 and during the whole series between 12° C. and 16° C, but did 

 not fall below, nor rise beyond these limits. During each mea- 

 surement, the water was kept circulating in the way described, 

 and equality of temperature of the platinum wire was secured by 

 uninterruptedly fanning it. 



By these arrangements were gained some important advan- 

 tages : for example, all reductions for temperature and their 

 concomitant errors were dispensed with, the thermo-currents 

 (the unavoidable result of the employment of ice) in the systems 

 were avoided, and the mercury was not required to be more than 

 ordinarily pure. 



When the currents of the system were balanced (that is to say, 

 when no effect was perceptible on the galvanometer by closing 

 the battery circuit), if the contact-rollers were moved 0*1 millim. 

 out of their place, the galvanometer mirror showed a deflection 

 represented by 5 millims. of the reflected scale passing by the 

 fibre of the telescope. 



The copper wire used in making the connexions between the 

 different parts was above 8 millims. diameter. It was softened 

 and the ends amalgamated. The same length was inserted in 

 the circuit of each tube (shown in the figure by dotted lines). 



of the nonius was placed on the 500 line, when, with x=Wj, no deflection 

 was observed by inverting these resistances. The wire had been drawn 

 with great care through stone, and its conicalness was entirely inappreciable. 



