122 
MESSRS. J. HOPKmSOX AND E. WILSON ON THE 
TliC Contact Apparatus .—Tins consists of a wooden pendulum carrjdng lead 
weights iCj, which were not moved during the experiments. The pendulum is 
released from the position p by the withdrawal of a brass plate, and, swinging- 
forward, strikes a small steel contact piece f, carried on a pivoted arm of ebonite. 
The initial position of this ebonite arm is determined by a contact pin e, about 
inch diameter, contact being maintained by a spring rn with an abutting rod 
insulated from a brass supjoorting tube by means of gutta-percha. This insulated 
rod is continued by a copper wire to the insulated pole of a quadrant electrometer Q. 
The brass supporting tube is continued by means of a metallic tape covering on the 
outside of the insulated wire, and is connected to the case and other quadrant of 
the electrometer. J.f, then, the pendulum be released from position ^9, the time which 
elapses between the termiiial piece g first touching the plate f, and the time at 
which contact is broken between e and the insulated stop is the shortest time we 
have been able to employ in these experiments, its duration being -00002 second. 
For longer times an additional device, showm in plan only, is used. It consists of 
a brass pillar li, which carries a steel spring S, and which is moved to and fro in 
V-shaped slides by means of a screw provided with a milled head n. which is 
divided into twenty equal parts on the outside surface. A pointer fixed to the 
frame indicates the position of the head, and a scale on the brass slide shows the 
number of revolutions of the head from zero position. The pendulum steel piece g 
is of sufficient width to touch the spring S as it moves forward and strikes the 
plate f. The zero of the spring S is determined electrically by moving- forward the 
pillar /q and noting the position of the nulled head when contact is first made, the 
steel piece g being in contact with f, but not disturbing its initial position. The 
plate jf is connected by a flexible wire with the slides which are in connection with 
the spring S through its support h. When, therefore, the spring S leads the plate f 
by any distance, the time of contact is that time wdiich elapses between g first 
striking S and the severance of contact between the pin e and its stop, always 
supposing- that g keeps in contact with S. A good deal of trouble was experienced 
before making this contact device satisfactory. The ebonite arm carrying- e and f 
was originally of metal,/"being insulated; but inductive action rendered the results 
untrustworthy. Then again, the spring- S, when first struck by the pendulum 
evidently again severed contact before f was reached. To get over this difficulty a 
subsidiary series of fine steel wires were attached to S, so that as the pendulum 
moves forward the wires are one after the other struck. In order that the pendulum 
should mT foul these wires or the spring S on its return to position p, it was slightly 
pressed forward by the hand at its central position. 
The method adopted is that of the hridge. Starting from mercury cups x, we 
proceed by a fine wire to the terminal /, and theiice, b}- a wire passing- down 
the pendulum, to g. From g we pass through spring S and tlie piece f dui-ing 
contact to one end of tlie bridge. Tlie flask F, or condenser to be experimented 
