118 Lord Kelvin on 



of contact electricity, the following description of my method 

 may possibly prove useful to experimenters. The same 

 method has been used to very good effect, but with a Bohnen- 

 berger electroscope instead of my quadrant electrometer, in 

 researches on contact electricity by M. H. Pellat, described 

 in the Journal cle Physique for May 1880. 



The apparatus used in these experiments was designed to 

 secure the following conditions : — To support, within a metallic 

 sheath, two circular discs of metal about four inches in dia- 

 meter in such a way that the opposing surfaces should be 

 exactly parallel to each other and approximately horizontal, 

 and that the distance between them might be varied at 

 pleasure from a shortest distance of about one-fiftieth of an 

 inch to about a quarter or half an inch. This part of the 

 apparatus I have called a " Volta-condenser." The lower 

 plate, which was the insulated one, was fixed on a glass stem 

 rising from the centre of a cast-iron sole plate. The upper 

 plate was suspended by a chain to the lower end of a brass 

 rod sliding through a steadying socket in the upper part of the 

 sheath. An adjustable screw on this stem prevents the upper 

 plate from being let down to nearer than about one-fiftieth 

 of an inch, or whatever shortest distance may be wanted in 

 any particular case. A stout brass flange fixed to the lower 

 end of this rod bears three screws, one of which S is shown in 

 the drawing, by which the upper plate can be adjusted to 

 parallelism to the lower plate. The other apparatus used 

 consisted of a quadrant electrometer, and in my original 

 experiments an ordinary Daniell's cell, in my later ones a 

 gravity Daniell's cell of the form which I described in 

 'Proc. R. S./ 1871 (pp. 253-259), with a divider by which 

 any integral number of per cents, from to 100 of the electro- 

 motive force of the cell could be established between any two 

 mutually insulated homogeneous metals in the apparatus. 



Connexions. — The insulated plate was connected by a brass 

 wire passing through the case of the Volta-condenser to the 

 electrode of the insulated pair of quadrants. The upper plate 

 was connected to the metal sheath of the Volta-condenser, 

 and to the metal case of the electrometer, one pair of quad- 

 rants of which were also connected to the case. One of the 

 two terminals of the divider, connected to the poles of the 

 cell, was connected to the case of the electrometer. To the 

 third terminal (the bar carrying the slider) was attached one 

 of the contact wires, which was a length of copper wire 

 having soldered to its outer end a short piece of platinum. 

 The other contact surface was a similar short piece of platinum 

 fixed to the insulated electrode of the electrometer. Hence 

 it will be seen that metallic connexion between the two plates 



