Variations in the E.M.F. of Clark Cells icith Temperature. 373 



be 107*15 ohms. Plugs would be taken out of the box R>i to the value 

 of 107 ohms, and the point P ou the wire would be selected such 

 that AP equalled O'lo ohm. The current through the wire supplied 

 by a storage cell, C, was then adjusted by the auxiliary resistance 

 B/2 until the standard E was balanced between the points H and P, 

 or so nearly balanced that no appreciable error would result. In this 

 way the wire was rendered direct reading for any subsequent com- 

 parisons, J cm. on the scale corresponding with one ten- thousandth 

 of a volt. A mercury cup was attached to the terminal of Hi ; it 

 was therefore possible to join E with the end of by simply dip- 

 ping a wire into the mercury. Thus, any change of resistance in the 

 main circuit by different pressure of the terminal, &c, was elimi- 

 nated, and the value of the current could be readily checked at any 

 time without interrupting the main circuit. This was done at fre- 

 quent intervals throughout every experiment. As a rule, very little 

 variation was found to take place. The value of the E.M.F. of the 

 standard cell was, of course, previously known i by an absolute deter- 

 mination. 



The cells under test were always connected with the point B of 

 the wire (1 metre from A), and were balanced against the standard 

 with never less than 10,000 ohms in series. The cells were placed in 

 a water bath, having room for five ; the positive poles were joined to 

 a common terminal, the negatives being led to separate mercury cups. 

 Two stirrers were provided for keeping the temperature of the water 

 uniform. There was also an outer bath containing the bath just men- 

 tioned. This outer one was heated when required by passing steam 

 through a piece of tubing immersed in the water, and cooled when 

 necessary by means of ice. 



The temperature of a set of cells was generally varied through a 

 considerable range, and brought back to practically the same value 

 as it had at the beginning of the experiment. Supposing no lag to 

 exist, the curve connecting temperature and E.M.F. would be the 

 same for both rising and falling temperature. But if there is a lag 

 of short duration {i.e., not cumulative), then, provided that the tem- 

 perature varies at the same rate throughout, two distinct curves, 

 parallel to the first for the greater part of their length, will result, 

 and will enclose an area between them. 



Further, if there is any semi-permanent lag, cumulative in its cha- 

 racter, the form of curve shown at pq (fig. 3) would result, the up 

 and down curves being parallel to one another, but inclined to the 

 true direction pq* This assumes that the lag has the same value 



* When the temperature rises the E.M.F. falls and the E.M.F. differences 

 increase. In all curves between temperature and E.M.F. differences the axis rela- 

 ting to the latter has therefore been drawn downwards. The origin for E.M.F. is 

 of course at a different point from that for E.M.F. differences, but the former is 



