178 PHYSICS. 
The connexion between the two mercury cups is shown more clearly by 
jig. 16. A metal ring surrounds one of the three pillars supporting the 
coil; to it is fastened the mercury cup, d, as also a copper wire, mnoc, 
which passes under the coil and ends at c in a little hammer of platinum. © 
The latter rests on a platinum plate which is soldered to a copper wire 
leading to the mercury cup, 6. The upper of these two wires has a thin 
place at n, where it is hammered flat; about this, the wire extremity, oc, 
can move up and down; on lifting the right extremity the hammer, c, is 
raised, and the current of course interrupted. The apparatus itself 
produces the rising and falling of the hammer. Thus the induction reel 
contains inside of it a holler cylinder” of: soft iron, becoming magnetic 
whenever a galvanic current traverses the primary coil. It then lifts an 
iron plate, attached at o to the upper wire, and consequently raises the 
hammer, c. The current is instantly broken at c, the iron cylinder loses its 
magnetism, the iron plate, and with it the hammer, c, falls, and the circuit 
is again restored. The cylinder again becomes magnetic, the iron plate is 
lifted a second time, &c., and the same actions are thus repeated as long as 
the battery continues to work. The setting screw, r, by which the wire 
rp can be raised or lowered, and with-it gp and con, is intended to regulate 
the distance of the iron plate from the electro-magnet, and with it the 
rapidity with which the interruptions of the current shall succeed each other. 
To produce the greatest possible effect on the nerves by the induced 
current, the metal cylinders, A and B, are fastened to the extremities of the 
secondary coil. These are to be grasped by the moistened hand, or filled 
with salt water into which the finger is to be dipped. A constant battery 
serves best for producing the current. 
The mutual influence exerted by the windings of one ana the same coil 
on each other fall properly under the head of induction phenomena. If a 
simple circuit be closed by a short wire, only a feeble spark will be obtained 
on opening it; this will nevertheless be much stronger if a long wire is 
used, and especially if the wire (insulated) be wrapped into a close coil. 
To take the shock conveniently and in quick succession, the apparatus of 
Neef (pl. 22, fig. 15) may be employed, without using the induced or 
secondary coil. Take two copper wires ending in metal cylinders, and dip 
the one into the mercury cup b, the other into the cup d, and grasp the 
cylinders with the hand. A violent shock will be felt at each opening of 
the circuit. This is illustrated by fig. 17. Here q represents the battery ; 
from one pole, when the circuit is closed, the current passes first to the 
cup 6, then over the interval, c, to the second cup, d, and from this through 
the spiral, s, to the other pole. When the circuit is opened at c, the shock 
passes through the human body connecting the cylinders A and B. 
Finally, the action of the apparatus of Neef may be intensified to a great 
degree by combining the two coils into one. For this purpose binding 
screws are attached to the extremities of the secondary coil at & and J, and 
into these are to be fastened the extremities of the primary coil, a and g ; 
¢ into 7 and a into k. 
F'cctrical currents are produced by magnetism. To show this fact, 
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