224 



Mr. G J. Burch. Contributions to a 



[Apr. 17, 



In dry weather the electrometer responds so readily to frictional 

 charges that the greatest care has to be taken to avoid touching the 

 table with the sleeve or coat, lest the mercury should be driven out 

 at the tip of the capillary. 



The cause of the leakage is obviously two-fold. Part of it is ex- 

 ternal, as is evident from the marked influence of the weather. The 

 capillary electrometer is necessarily a difficult instrument to insulate. 

 Glass is under the most favourable circumstances liable to attract 

 moisture in a rising temperature, and it becomes still more so when a 

 portion of its surface is in contact with sulphuric acid diluted to such 

 an extent. Varnish cannot be used, but I have thought vaseline round 

 the ends of the tubes diminishes the leakage. 



The other part of the leakage is internal. The cause of it is easily 

 explained. The acid wets the glass and the mercury does not. There 

 is therefore a tendency for the acid to insinuate itself between the 

 mercury and the walls of the capillary. That it does so is evident on 

 examining the mercury column under the microscope by front light 

 with a high power. A film of liquid can be seen between the metal 

 and the glass, and traced for some distance. When a short column of 

 mercury 2 or 3 cm. long is used, this film soon passes right up the 

 tube, and in a few weeks acid can be seen above the mercury. Even 

 in the U-tube, where the mercury is beneath the acid, the same action 

 takes place, though less rapidly. For this reason I prefer to have a 

 depth of 8 or 10 cm. of mercury in the capillary, and 5 cm. in the 

 U-tube. The internal leakage is least when the electrometer is new. 

 Instruments which leak badly generally also " creep," i.e., if left on 

 open circuit the mercury does not remain at zero, but creeps slowly 

 up or down, owing to some electomotive force within the instrument. 



It seems at first sight inconceivable that there should be no current 

 through a circuit consisting of platinum — mercury — dilute sulphuric 

 acid — mercury — platinum, all of which are good conductors. Yet we 

 have this fact : the time of half-charge of a quick electrometer is of 

 the order of second, but the time of half-discharge of the same 

 instrument on open circuit may be counted by days. The resistance 

 to the passage of the current is manifestly not ohmic, but some effect 

 produced at the interfaces between mercury and dilute acid. 



The conditions suggest the counter electromotive force of polarisa- 

 tion, and here we are met by the experiments of Bouty, showing that 

 the sum of the electromotive forces of polarisation at the two electrodes 

 is always less than the applied electromotive force, no matter how 

 weak it may be, so that there is always a permanent current through 

 the electrolyte. 



The explanation is I believe to be found in the fact that we are 

 dealing with an interface between two liquids which cannot diffuse 

 into each other, and that the electrical and chemical as well as 



