Contact Electricity of Metals. 90 



electrometer. But the test would be difficult, because of the 

 difficulty of preparing the opposed surfaces of two equal and 

 similar disks, so as to make them equal in their surface-Volta- 

 potentials within one one-thousandth of a volt, or even to 

 make their difference of potentials constant during the time of 

 experiment within one one-thousandth of a volt. There 

 would, however, be no interest in making the experiment in 

 this way. because by the electromagnetic method we can with 

 ease exhibit and measure with great accuracy the difference 

 of potentials between A and B, by keeping them exactly at 

 one temperature and connecting them by wires of any kind 

 with brass or other terminals of a galvanometer of high 

 enough resistance not to sensibly diminish the difference 

 of potentials between A and B, provided all the connexions 

 between metals of different quality except J and K are kept 

 at one and the same temperature (or pairs of them, properly 

 chosen, kept at equal temperatures). 



§ 20. Suppose, now, instead of breaking a circuit of two 

 metals at a place in one of the metals, as A B in copper 

 m fig. 9, we break it at one of the junctions between the two 

 metals, as at C C, I' I, tig. 10. C D represents a movable 



Fig'. 10. 



slab of copper which (for §22 below) may be pushed in so as 

 to be wholly opposite to I' I, or at pleasure drawn out to any 

 position, still resting on the copper below it as shown in the 

 diagram. Calling zero the uniform potential over the sur- 

 faces C CD, the potential at V I would be about +'16 volt 

 (according to Murray's results for emerv-polished copper and 



H2 



