142 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
The guard ring is supported by three strong legs, C, C, made of zinc, and provided 
with good levelling screws and jamb nuts, D, D. The ring is put to earth bv a 
soldered connection, which ultimately is soldered to a water-pipe. The high poten¬ 
tial plate E is a large glass disc, 30‘5 centims. in diameter, and optically flat. It was 
originally silvered, but the silvering having gradually tarnished by contact with 
sulphur, and its conductivity having become unreliable, it was replaced by some good 
thin tin-foil,fastened down with thin starch paste, and put on so well that no appreciable 
deviation from flatness could be detected. It was attached by a clamp and wire, H, 
to the high potential end of the transformer. The thickness of the plate, 3'3 centims., 
gets rid of any fear as to flexure producing an appreciable effect. 
This plate was supported by three ebonite cones, J, J, 2 centims. in diameter at the 
thick part, and 5'4 centims. long. These cones were got to insulate well, they were 
supported by a wooden stand, K, triangular in shape, and carried in turn on three 
levelling screws, working through brass fittings, and bearing on brass caps supported 
by three of the double glass and paraffin insulators, M, M, M, we have had occasion to 
refer to so often before. All this part of the apparatus requires to be well and 
substantially made, or it tends to tilt over a little. The top of the glass disc was 22'5 
centims. above the slate bench. The wire connecting the tin-foil to the transformer 
was short and thick, but it was found that no alteration was made by twisting it into 
a spiral, so that no “ resonance ” effects are to be feared. 
Screens of tin-foil pasted on to cardboard completely surrounded all parts of the 
apparatus. The first screen stood on the guard ring and enclosed the back of the sus¬ 
pended plate, allowing the wire to pass through a small hole. Outside the whole affair 
was another and larger screen. Both these screens were cylindrical and properly 
shaped, and were made in two parts joined by tin-foil flaps. The balance was also 
completely encased in a screen of tin-foil and gauze, allowing the pointer to be observed 
with ease. The panels of the balance case were separately screened by gauze to allow 
of the adjusting of weights, &c. The elementary mistake of having bad screen connec¬ 
tions was not made. Screens are very necessary to prevent air currents, as well as to 
hinder electrostatic action. The balance was in a room with a south aspect at the 
opposite'end of our building to the room occupied by machinery. 
Balance .—This was a rather short arm balance, which was formerly used for 
weighing quartz, &c. It has a high range of sensiti veness, though this was diminished 
to a convenient amount in order to secure other advantages'. It was furnished with 
an adjustable screw stop at B, by Mr. Allen. The balance will carry 1 kilogram, so 
that there is no danger of warping. 
Electro-Dynamic Action.— -This was tested by insulating the glass plate, the other 
plate being connected to earth on one side, and to the terminal of the transformer on 
the other. No trace of any action could be detected, i.e., either nothing at all or else 
something incomparably smaller than we could approach in the experiments on 
weighing. 
