MAGNETISM AND ELECTRICITY. 161 
pile is preserved in a glass tube, and such pressure exerted upon its top as 
to maintain the close contact of all the disks, which must be very 
numerous. The action of this pile, although very slight in comparison 
with the wet pile, remains constant for months, and even years, on which 
account it also may be called a constant battery. 
One of the most important applications of the dry pile is in the 
electrometer of Bohnenberger. This is a gold leaf electrometer with, 
however, but one leaf, towards whose two sides opposite poles of two dry 
piles are turned. If the least electrical charge be communicated to the 
gold leaf, which, protected from the air, hangs perfectly quiet when 
uninterrupted, it will cause the leaf to move to one side or the other. In 
this manner the character of the electricity imparted can be readily 
ascertained from the pole, ¢, towards which the leaf inclines. Positive 
electricity, of course, turns towards the negative pile. In the improvements 
of Becquerel and Fechner (pl. 20, fig. 50), a dry pile of 800 to 1000 
plates, inclosed in a glass tube, is placed horizontally in a box. The tube 
is capped with brass at each end, as seen in fig. 51. The caps 
communicate conductively with the poles of the pile, and from them pass 
the wires, p and f, terminated by the polar plates, z and y. The signs 
+ and — are placed on the upper surface of the box from which the poles 
vroject, to indicate their electrical character. 
3. The Action of the Galvanic Current. 
As before remarked, a galvanic current is first started when the two 
poles of a galvanic battery. in working order, are united by a conductor. 
If the extremities of the two polar wires (pl. 20, fig. 49) are brought to 
within a short distance of each other, a spark will be seen to pass between 
them. By interposing different substances in the current between the 
poles, very striking and varied electrical effects will be produced. These 
may be divided into physiological, chemical, and physical. Omitting for 
the present any mention of the first class, chiefly exhibited in the nervous 
convulsions of muscular fibre, we pass to the second, the chemical, which 
consist in the decomposition of water, and of various other compound 
bodies. Thus water is decomposed by the galvanic current into oxygen 
and hydrogen, an experiment which the apparatus represented by 
fig. 64, pl. 20, is well calculated to exhibit. This consists of a wine-glass, 
at the bottom of which two platinum wires, f and f’, are melted in; above 
these stand two small glass receivers, 0 and h, which have been filled with 
water and inverted in the wine-glass. On bringing the wires, f and f’, in 
communication with the poles of a galvanic battery, bubbles of gas will be 
developed, oxygen rising to the top of the receiver, over the positive pole, 
and hydrogen over the negative. If the separation of the gases be not 
necessary, the apparatus represented in fig. 65 may be employed. Here 
the polar extremities are formed by two large plates of platinum, on which 
the decomposition of the water takes place, the gases ascending to the top 
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