138 EEPORT— 1892. 



APPENDIX II. 



On the Change of Resistance of Mercury tuith Temperature. 

 By M. G. GuiLLAUME. 



At the meeting of the Committee M. Guillanme communicated the 

 results of his determination of the relation between the resistance of 

 mercury and the temperature. Great precautions were taken with the view 

 of ensuring that the whole of the mercury in the tube should be at the 

 temperature of the bath. 



Two series of determinations with different arrangements in the 

 bridge were made. The results of these two series give for the resistance 

 of mercury in a glass tube in terms of the temperature the values — 



(a) Rt= Ro (1 + •00088023T + -OOOOOlOOeST^) ; 

 (6) Rt=Ro (1 + -00088157T + -000000990912). 



And for the specific resistance of mercury the values — 



{a) p,=po (1 + -00088745T + -00000018112) ; 



(b) p,=po (1 + -OOOSSSrOT + •0000010022T2). 



In the formulae T is the temperature reckoned from freezing-point by the 

 air thermometer. According to Mascart, de Nerville, and Benoit — 



Rt=Ro (1 + -0008649T+-00000112T2) ; 



while according to Strecker — 



Rt=Ro (1 + -0009001 + -00000045T2). 



APPENDIX III. 



On a Special Form of Clark Cell. By Professor H. J. Carhart. 



Portahility. — Standard cells must be portable in order to make them 

 serviceable for general, technical, and scientific purposes. To secure 

 portability I have adopted the following construction : Into the bottom of 

 a glass tube | in. x 2\ in. is sealed a platinum wire. In filling pure dis- 

 tilled mercury is first poured into the tube. On this is placed the 

 mercurous sulphate paste. A tightly fitting cork diaphragm is then 

 pushed down firmly upon the paste. Some zinc sulphate solution is 

 then poured in, and a zinc rod is immersed in this solution, its lower end 

 touching the cork. The tube is then securely sealed. Such a cell is 

 perfectly portable, and may be sent by post without disturbance to its 

 contents. 



Temperature Coefficient. — It is well known that an increase in the 

 density of the zinc sulphate solution decreases the E.M.F. of a Clark 

 cell. This effect is included in the temperature coefficient of a Clark 

 cell containing crystals of zinc sulphate, since some of the crystals 

 dissolve when the tempei-ature rises and the density increases. When 

 the temperature falls recrystallisation occurs. To avoid the change in 

 E.M.F. due to this change in density I have preferred to use a solution 

 saturated at 0° C. Such a solution I have found to have a specific 



