ON GASEOUS EXPLOSIONS. 183 
of the stiff springs S, and S, rest on the fibre cams. A projection 7, 
carrying a platinum-pointed screw p is riveted to one of the springs, 
and the-screw p is adjusted so that its point is just clear of the platinum 
rivet in the other spring when both springs are riding on the circular 
surfaces of their respective cams. Contact is made when the rotation 
of the lay shaft in the direction of the arrow brings the radial step 
w, of the cam W, under the spring S., thereby allowing it to fall down 
the step, thus bringing p and r together. Contact is broken when the 
radial step w. of the cam W; reaches the spring S:, thereby allowing 
the second spring to fall down the step w.. The epoch and duration 
of contact are readily adjusted by adjusting the angular positions of 
the cams relatively to the bush and also with regard to one another. 
The distances between the springs and the platinum contacts and the 
steps w are exaggerated in the diagram in order to make the principle 
of the apparatus clear. The percussion form of contact with platinum 
points is found to give definite and certain results. The contacts keep 
fi 4) t 
“CLAMPING Screw 
FIKREO PLATE 
Fia. 4. 
clean, and no trouble of any kind is experienced with them. The general 
arrangement of the electrical connections are shown in fig. 4. In this 
figure PS, QS are the equal ratio arms of the Wheatstone bridge. 
The galvanometer G is connected to the point § and to the sliding 
contact on the bridge-wire BW. The thermometer and its leads P 
are connected on one side of the bridge-wire, and the compensator C 
and the balancing resistance R on the other. The battery circuit 
includes a mercury reversing key K, an adjustable resistance r, and 
a storage cell V; and the battery is connected to the bridge at the 
points P and Q, and to the brushes of the periodic contact-maker at E. 
The brushes E are carried by an insulated arm A bolted to a divided 
dise O riding loosely on the lay shaft of the engine, and capable of 
being clamped in any position by the screw L. The index I shows 
the crank-angle corresponding to the middle point of the contact when 
the insulated copper strip D carried in thé fibre bush F passes under 
the brushes, 
