516 Determination of Electromotive Forces of Voltaic Cells. 



other electromotive force. When, as in the case of Clark- 

 standard cells, the cell is incapable of maintaining a current, 

 a high-resistance galvanometer or an additional resistance 

 should be included in its circuit. If very great sensibility is 

 not required, a quadrant-electrometer may in such cases be 

 substituted for the galvanometer. The standard resistance 

 must be of such a form that no sensible error is introduced 

 through heating by the passage of the current. A good plan 

 is to wind well-insulated platinoid wire in one layer on the 

 outside of a brass or copper cylindrical vessel which can be 

 filled with water. The temperature of the water, when it is 

 nearly the same as that of the air outside, will be very ap- 

 proximately the temperature of the coil. For still greater 

 accuracy the cylinder may be fitted with a jacket and im- 

 mersed in a vessel of water, and appliances introduced for 

 changing the water in each part and keeping account of its 

 temperature. For use with the deci-ampere balance, a pla- 

 tinoid resistance of two ohms is sufficient for any single cell. 

 A resistance of two ohms, made of insulated platinoid wire one 

 millimetre in diameter, and wound on a brass tube capable 

 of holding half a litre of water and simply exposed to the air 

 outside, will carry a current of one ampere for an hour with- 

 out changing its resistance more than a tenth per cent. The 

 water should be stirred when the readings are taken, and, if 

 necessary, the change of resistance can be approximately 

 allowed for by taking its temperature. To measure by means 

 of the deci-ampere balance an electromotive force of from 

 one to two volts, a battery of two small secondary cells or four 

 Daniell cells, a resistance of two ohms such as has just been 

 described, with the other appliances as indicated in the figure, 

 are all that is necessary. 



The method has been applied in my laboratory by Mr. 

 Thomas Gray for the measurement of the electromotive forces 

 of standard and other cells, and has been found very con- 

 venient. The results obtained for four Clark-standard cells 

 set up by Mr. J. T. Bottomley in March last were almost 

 identical with one another, and gave 1*439 Rayleigh, or 

 1*442 legal, volts at 11° C. The variation of the electro- 

 motive force of these cells with temperature has not yet been 

 determined ; but assuming the average value obtained for this 

 variation by Lord Rayleigh, namely a fall of *077 per cent, 

 per degree centigrade rise of temperature, and correcting to 

 15° C, we obtain slightly under 1*436 Rayleigh volts at that 

 temperature. This result is interesting as showing a difference 

 of less than -j^th per cent, from that obtained by Lord Eayleigh 

 for similar cells, which was 1*435 at 15° C. 



