ELECTROMETERS. 231 



A stiff piece of platinum wire attached to the needle at 

 right angles to it hangs or projects vertically down ; and from 

 the lower extremity of this a very fine piece of platinum 

 wire hangs down and dips into the strong sulphuric acid which 

 forms the interior coating of a Leyden jar. The case of 

 the instrument is of glass, with slips of tinfoil pasted over 

 it to protect the interior from external influence. There are 

 other particulars to which I need not allude just now, as 

 I shall have to speak of them in connection with the 

 quadrant electrometer, a development from that which is 

 before us at present. 



Before the Leyden jar with which the needle is connected 

 is electrified, the needle is adjusted mechanically, so that 

 it projects over one of the divisions between the two halves 

 of the divided ring. Now if the two halves of the ring 

 are at the same potential, both being, let us suppose, con- 

 nected with the case of the instrument, and if the parts of 

 the instrument are quite symmetrical, then when the Leyden 

 jar and needle are electrified, no disturbance of the needle 

 will be experienced. But let one of the half -rings be con- 

 nected with a body to be tested whose potential differs 

 from that of the case of the instrument to which the other 

 half-ring is connected, and there will plainly be attraction 

 or repulsion of the needle, according to the nature of the 

 electrification of the tested body, and the needle will move 

 towards one half-ring or towards the other. 



Now let me in conclusion refer to one very remarkable 

 application that Sir William Thomson has made of this 

 electrometer. It was chiefly for that purpose that I have 

 brought it before you, because the instrument itself, though 

 in earlier times it did admirable service as an electrometer, 

 has now been superseded by the quadrant electrometer. 

 By means of this instrument Sir "William Thomson was 

 able to furnish a test between Volta's contact theory of his 

 pile and the rival chemical theory. The fundamental 

 statement of Volta's theory was that a piece of metallic 

 zinc and a piece of metallic copper put in contact assume 

 different potentials, the zinc becoming positive with respect 

 to the copper. The supporters of the chemical theory 

 denied this, and explained away the experiments that were 

 adduced in support of it. It is now, however, regarded as 

 established, and here is one of the experiments of Thomson 



