Electromotive Force of different Forms of the Clark Cell. 121 



for the earlier comparisons was a long wire, having a resistance of 

 86 ohms, wound on a cylinder in one hundred turns. Each turn 

 was divided into one hundred parts, and readings were taken to one 

 tenth of a division. Each division corresponded approximately to one 

 five-thousandth part of a volt, a storage-cell being used to supply 

 the steady current through the wire. This potentiometer had been 

 accurately calibrated throughout its length at two different dates. 

 The results agreed so closely that it could be used with confidence 

 for measuring relatively large differences of potential with an 

 accuracy of at least one-half division of the wire, equivalent to 

 0*0001 of a volt. The errors of the uncorrected wire amounted to 

 over ten divisions in many places. 



.For the later experiments a simpler, and in many respects more 

 convenient, form of potentiometer was used. Two resistance boxes, 

 containing resistances adjustable up to 2000 ohms each, were con- 

 nected by a platinum-silver bridge-wire having a resistance of 

 18 ohms. The wire was 2 metres long, in four lengths of 50 cm. 

 each, with a millimetre scale, and was adjusted to read direct in 

 volts, at the rate of 1 mm. to one-hundredth of a millivolt, in the 

 following manner : — A resistance of 18/20 x 1420 ohms was taken 

 out of the first box, and the resistance in the second box was 

 adjusted to make the standard cell at 15° C. read near the point 

 140 cm. of the wire, i.e., 14 millivolts above T420 volts. The 

 bridge-wire could thus be used directly for measuring small diffe- 

 rences of E.M.F. not exceeding 20 millivolts, with an accuracy of at 

 least one part in a thousand on a difference of this order. Larger 

 differences could be readily dealt with by transferring resistance 

 from one box to the other in such a way as to keep the sum con- 

 stant, each ohm transferred being reckoned at 20/18 of a millivolt. 

 It will be understood that the resistance of the bridge-wire was 

 carefully measured in terms of the boxes, that their temperature- 

 coefficients were very nearly the same, and that the wire was tested 

 for uniformity, to insure the above order of accuracy in the deter- 

 mination of differences of E.M.F. of this magnitude. 



§ 5. Thermometry. 



In working to the hundredth of a millivolt, it was necessary to 

 Know the temperature of the baths to the hundredth of a degree C. 

 Two thermometers were generally used, one in each water-bath. 

 They were both carefully compared with a platinum thermometer, 

 and their indications were in all cases reduced to the absolute scale. 



One of the thermometers was by Geissler, divided to tenths of a 

 degree. This thermometer had evidently been graduated to read 

 temperature on the absolute scale direct. Its errors, after correcting 



