418 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
three screws upon a light brass triangle hanging in the balance by a stout copper 
wire. The fixed coils are those of the dynamometer, described in Maxwell’s 
‘Electricity,’ § 725, and in Latimer Clark’s paper (Phil. Trans., 1874, Part I). In 
setting up the apparatus the ebonite coil is first suspended, and the dynamometer 
coils are levelled, and adjusted laterally until concentric with it. This is tested by 
carrying round a metal piece making five contacts with the upper ring of the 
dynamometer, and provided with a pointer just reaching inwards to the circumference 
of the ebonite coil. The piece in question may be described as a sort of three-legged 
stool, standing upon the upper horizontal face of the dynamometer ring and carrying 
below two studs which are pressed outwards into contact with the inner cylindrical 
face of the ring. As the piece is carried round the pointer describes a circle coaxal 
with the dynamometer rings. To level the ebonite ring, the distance is calculated by 
which its upper surface should be below the upper surface of the (upper) dynamo¬ 
meter ring, and a pointer attached to a straight rule is so adjusted that when the rule 
is laid upon its edge along the upper face of the dynamometer ring the pointer should 
just scrape the upper face of the ebonite ring. B} 7 " applying this test at three points 
the ebonite ring is brought to occupy the desired position. These adjustments were 
made in the first instance by our assistant, Mr. G. Gordon, and subsequently 
examined by ourselves. With a little care the necessary accuracy is attained without 
difficulty, for, it is scarcely necessary to say, all the errors due to maladjustment are 
of the second order. When in use the suspended parts are protected from currents 
of air by a suitable paper casing. 
Examination showed that the insulation of the various parts was satisfactory. 
Twenty-five cells of a De La Rue’s battery failed to show any appreciable leakage 
between the wire and the rings of the dynamometer coils, though the capacity of the 
condenser thus formed was very noticeable. 
§11. The test for leakage from winding to winding of a coil is a more difficult 
matter. The ebonite ring was first wound on August 9, 1882, and its galvanometer 
constant was compared with that of one coil of the dynamometer by Mr. J. M. 
Dodds. The result agreed very ill with the measurements taken during the winding, 
and led to the suspicion that several turns were short-circuited by a false contact. 
The matter was put to a further test in two ways. A second coil of the same 
dimensions was wound with the same number of turns; and the two coils were 
placed co-axally close together, and so connected in series that a current would 
circulate opposite ways. The circuit was completed by a galvanometer of long 
period. Under these circumstances when one pole of a very long steel magnet is thrust 
suddenly through the opening, there should be no effect observable if the insulation 
is good; but if any of the turns of one of the coiJs are short circuited the ofher coil 
will of course have the advantage, and the galvanometer will indicate a current in the 
corresponding direction. It was found in fact that the second coil preponderated, and 
that 13 extra turns had to be put upon the first coil to obtain the balance. With 
