224 



Profs. W. E. Ayrton and John Perry. [Mar. 21, 



to the roller, readings may be made in the ordinary way. Such a roller 

 and clock costs very little, and form a valuable addition to every re- 

 flecting galvanometer or electrometer. 



Paraffin Wax. 



The plate A being zinc and connected with the insulated electro- 

 meter-electrode, and the paraffin being solid between the two plates, 

 we first short-circuited the arrangement to get zero, then insulated and 

 allowed the electromotive force of the paraffin cell to charge plate A, 

 and the electrometer quadrants in connexion with it. 



A detailed account of our experiments on conductivity is given 

 further on. It is sufficent here to state that at low temperatures the 

 conductivity of paraffin wax is exceedingly small, the conductivity in- 

 creases to the melting point, the increase being extremely rapid during 

 melting, and there seems to be a regular increase during the melted 

 state. Hence in the experiments with the zinc plate at low tempera- 

 tures, the resistance is so great that although there is a slight charging 

 of the quadrants due to electromotive force, this is somewhat disguised 

 by defects of the electrometer. In fact, at low temperatures, it is im- 

 possible to measure the electromotive force of this simple cell, although 

 the existence of an electromotive force is quite evident, but when the 

 conductivity becomes sufficiently great, and this occurs long before 

 there is any appearance of melting on the upper surface of the paraffin, 

 the charge reaches a maximum with sufficient rapidity for its accurate 

 measurement. At a few degrees above the melting point the maxi- 

 mum charge established itself at once, and however often the plates 

 were short-circuited for the purpose of taking the zero, the maximum 

 charge was found to be approximately the same, and to correspond 

 with an electromotive force of 0'73 volt when least, and 0'75 volt 

 when greatest : it was also not much affected by change of tempera- 

 ture. To make these measurements we used as a standard a Latimer 

 Clark's constant cell.* 



When the plates are charged to their maximum difference of poten- 

 tials there is electric equilibrium, and no further current tends to pass 

 through the arrangement, the electromotive force being balanced. If 

 then E represents this maximum charge, and if Ei is the charge at a 

 time (£1), and E 2 the charge at the time t 2 , then — 



_JL_ log ^z?± 



to — 1\ Eo — E 2 



multiplied by a constant depending on the electric capacity of the 

 arrangement represents the conductivity. As the measurement of 

 any particular temperature is rather coarse, we cannot readily compare 



# In our last paper we gave 0*75 yolt as the correct measurement of the electro- 

 motive force of contact of zinc and copper. 



