638 
MR. H. B. DIXON ON CONDITIONS OF CHEMICAL CHANGE 
Ruhmkorff, and of a short brass tube and tap for admitting the gases. To the other 
end of the brass pipe was screwed the cap C containing the metallic bridge to be broken 
by the explosion : this consisted of a thin strip of silver foil soldered with fusible 
metal on to two insulated brass pieces let into two vulcanite plugs, one on each side of 
the cap. The silver strip was sufficiently thick to convey the current of one Grove 
cell without becoming heated. No fulminate or other explosive was employed. The 
silver strip was invariably broken by the flame. The distance from the platinum wires 
to the silver bridge was 1 '049 metre. In the circuit, of which the silver strip formed 
a portion, a chronograph was inserted. This consisted of two small electromagnets 
which, when the current was flowing, held down a style against a spring. The chrono- 
Arrangement of Explosion Tube. 
Plan of cap C. Section of cap C. 
graph was adjusted so that the end of the style pressed lightly against the blackened 
surface of a glass plate carried by a heavy pendulum. The pendulum, at the lowest 
point of its swing, overturned a brass upright moving on a horizontal axis, and so broke 
the primary coil of the Ruhmkorff, inducing a direct current in the secondary coil.' 
This induced current caused a spark between the platinum wires which fired the explo¬ 
sive mixture. Until the explosion reached the silver foil the style continued to 
describe an arc of a circle on the moving blackened plate. When the silver was 
broken, the style, released from the magnet, sprang upwards, marking the moment of 
interruption. To correct for the error due to the retardation of the chronograph a 
blank experiment was always made immediately before firing the mixture. The wires 
of the chronograph were attached to the break of the primary coil of the Ruhmkorff, 
so that the circuit was completed through this break instead of through the silver foil. 
