MAGNETISM AND ELECTRICITY. 027 
terminating above in a mercury cup. A horizontal wire ending in two 
balls plays on a fine pivot which rests in the bottom of the mercury cup; at 
the same time two short vertical wires dip into the acidulated water with 
which the copper is filled. A current thereby arises, which, for instance, 
ascends the vertical rod, traverses the horizontal arms in opposite directions, 
and descends into the acidulated water. If the vessel be surrounded by a 
wire coil, through which the current is passed, the horizontal wire will be 
set into rotation by the influence of the circular current. 
b. Phenomena of Induction. 
An electric current, as discovered by Professor Henry, can beget like 
currents in a neighboring conductor at the moment of the commencement 
or cessation of the former, or even by simple approximation or separation : 
these are called induced or induction currents. To exhibit these 
phenomena let two insulated copper wires be wound close together, 
without conducting contact, on a reel of wood or metal (pl. 22, fig. 14). 
Close the circuit of a battery with one of these wires by means of its 
extremities, a, 6. Simultaneously with the passage of a current through 
this wire, a current in the opposite direction will be developed in the other 
wire, provided its extremities, c and d, are connected, which should be 
lone by means of a multiplier. The multiplier will indicate by the 
deflection of its needle the existence of the current just referred to; this 
surrent will, however, at the instant the primary circuit is closed, be only 
momentary. On breaking the primary circuit the deflection of the needle 
will indicate a second current in cd, but in a direction opposite to its 
original one. 
These induced currents are capable of producing sparks, shocks, and, 
indeed, all the phenomena of the primary currents. On bringing the 
extremities, c and d, together (pl. 22, fig. 14), sparks will pass between 
them; take them in the hands and a shock will be felt at the closing or 
opening of the primary circuit. Ifthe secondary wires have a considerable 
length, the intensity of the induction current may even be much greater 
than that of the primary, for which reason an induction coil is well 
calculated for producing physiological effects. This is especially the case 
when the circuit can be closed and opened in very rapid succession, which 
is practicable by means of the apparatus constructed by Neef and Wagner. 
pl. 22, figs. 15 and 16. Fig. 15 represents an induction coil, as shown in 
fig. 14; the two wires are generally wound with differently colored silk 
for the sake of being more readily distinguished. One pole of the battery, 
the positive for instance, is connected wath a mercury cup by a wire, ab, 
this again being connected with a second cup, d. Into this latter cup dips 
one extremity of the inducing or primary coil, which enters the spiral at e, 
again leaving at f, and as the extremity, fg, is connected with the other pole, 
the primary current passes from a through b,c, d,e, f,tog. The secondary 
wire enters the coi! at h and emerges at7; its extremities are Al aud tk. 
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