104 



THE POPULAR EDUCATOR. 



VOLTAIC ELECTRICITY. I. 



DISCOVERY OF GALVANISM SIMPLEST MODES OF PRODUCING 



AN ELECTRICAL CURRENT DRY PILE BOHNENBERGER'S 



ELECTROSCOPE ACTION OF BATTERY. 



INTERESTING as is the department of electrical science which 

 we have already examined, we shall find that Voltaic Electricity 

 is still more so, on account of the much greater variety and im- 

 portance of its practical results. The effects that we shall 

 notiae, and the means of producing them, will at first sight 

 appear so different from those which we have been considering, 

 that we shall imagine that we are dealing with an entirely diffe- 

 rent agent, but as we advance we shall perceive that they are 

 but different manifestations of the same cause. 



This branch of the science is by far the most modern, dating 

 back only as far as the year 1790, at which time all the most 

 important experiments with frictions! electricity were well 

 known. Its first discovery may be attributed to accident. The 

 wife of Galvani, a professor of anatomy at Bologna, being 

 seriously unwell, was advised by her medical attendant to try the 

 curative effects of frog broth, a remedy which would be very un- 

 popular with most Englishmen. Some frogs were accordingly 

 prepared, and happened to be lying upon the table in his labora- 

 tory at a time when his assistant was working an electrical 

 machine near to them. It was then noticed that when sparks 

 were drawn from the conductor of the machine the limbs of the 

 frog were convulsed : this was seen more clearly when a piece 

 of metal was in contact with the nerve. Galvani was informed 

 of this fact, and, being ignorant of the principles of electrical 

 induction, at once commenced a series of experiments with a 

 view to ascertain the cause of the strange phenomenon. 



Some frogs' legs were accordingly prepared ; the lumbar nerves 

 being laid bare, they were then suspended by copper hooks from 

 an iron rod until wanted for experiment. There was no special 

 reason for choosing copper hooks, except that they happened to be 

 the most handy at the time. He noticed, however, that as soon 

 as the muscle of the leg came in contact with the iron bar, con- 

 vulsions were produced similar to those which had been noticed 

 before. This increased his surprise, and upon examination he 

 found that the hook touched the nerve, and at length he started 

 the theory that there was a natural flow of electricity from the 

 nerves to the muscles, and that when the circuit was completed 

 this current caused the contraction. He seems to have lost 

 eight of the fact of two different metals being required, and 

 thus his theory was altogether wrong. It gained ground, how- 

 ever, and was for some time universally received. 



Volta, who was a professor of natural philosophy at Pavia, 

 repeated the experiments, and found that when a rod composed 

 of only one metal was employed, no convulsions were produced, 

 while if the rod were partly composed of copper and partly of 

 iron, the effects were seen even if both ends were in contact 

 with the muscle. He thus perceived that the essential point 

 was the contact of two dissimilar metals, and that the muscle 

 and nerv j only served the part of a conductor and indicated the 

 passage of the current. 



The annexed figure (Fig. 1) will explain the manner in which 

 this experiment may be performed. When the legs with the lower 

 vertebrae are removed, the nerves will be seen in the form of two 

 whitish threads. One end of the rod, which should be composed 

 of a piece of copper and one of zinc joined together, is then 

 placed between the nerves and the bone, and when the other end 

 is brought into contact with the muscles of the leg, the limbs 

 will be at once drawn up as shown. 



Volta' s theory was that the electric current was produced 

 by the mere contact of two different metals, and though this 

 theory is now believed to be incorrect, yet there are several 

 3xperiments which appear to support it. Take two discs made 

 respectively of polished copper and zinc, and insulated by, means 

 of glass handles. Having pressed them together, separate them 

 rapidly, and touch one of them against the lower plate of 

 the condensing electroscope, the upper plate being connected 

 with the ground. Now remove the disc, and having touched it 

 with the finger to neutralise any charge, press them together 

 again and touch as before. After this has been repeated 

 several times, the condensing plate of the electroscope may be 

 lifted, and it will be found that a slight charge has been ac- 

 cumulated, causing the gold leaves to diverge. 



There are several simpler ways in which the production of 



electricity by the contact of two metals may be shown. If we 

 procure a small piece of zinc, and place it on the upper side 

 of the tongue, and also place a haK-crown or other piece of silver 

 under it, we shall notice a peculiar taste when the pieces of 

 metal are made to touch one another. This taste is unlike any 

 other, and will manifest the presence of a very faint electric 

 current. In setting up small batteries, the terminal wires are 

 frequently placed on the tongue to see if the battery is working 

 properly, and after a little practice a good idea may in this 

 way be formed of its power. 



If one of the pieces of metal be placed between the upper lip 

 and the gum, and the other under the tongue, the taste will be 

 noticed as before, and, in addition, a faint flash of light will 

 be observed when the experiment is tried in a darkened room. 

 This flash is seen only at the moment the contact is made. 

 The taste, however, remains as long as the metals touch one 

 another. 



If a piece of copper or a penny be laid on a sheet of zinc, 

 and a common garden snail be thea placed upon it, the snail will 

 draw in his horns and seem to shrink back when he comes 

 against the copper, and after touching it once or twice will 

 avoid it altogether, a faint shock being evidently produced when- 

 ever he touches it. As a practical application of this, it has 

 been suggested to place an edging composed of a strip of copper 

 with one of zinc soldered on to it round flower-beds, and in 

 this way to keep off slugs and snails. Wires of copper and 

 zinc are somewhat twisted round the bottom of poles which sup- 

 port dahlias for a similar reason, and are said to be effective. 



In pursuing his experiments on the electrical effects pro- 

 duced by contact, Volta invented the apparatus known after him 

 as the Voltaic Pile (Fig. 2). This consists of a series of discs of 

 copper, zinc, and cloth, the latter being sometimes cut a little 

 smaller than the metal ones. The whole are then arranged in 

 the following order : A disc of zinc, then one of cloth, which 

 is moistened with water rendered slightly acid by the addition of 

 sulphuric acid, and then one of copper; then zinc, cloth, and 

 copper again, and so on. Frequently the copper and zinc are 

 soldered together, so that then we place the compound discs with 

 the zinc side upwards, and lay between each a piece of cloth. 

 Glass rods are usually placed so as to keep them all in position, 

 and wires are brought from the end plates to form the terminals, 

 that leading from the copper plate being negative. 



If the pile be not in?ulated, the lower end is neutral, and 

 the rest of the pile is charged with free electricity, which is 

 positive if a copper plate be in connection with the ground, and 

 negative if it be one of zinc. AVe may, however, insulate it by 

 means of a cake of resin, and then each end will be charged. If 

 a pile of this kind be made sufficiently powerful, a feeble 

 spark will be seen between the wires when they are brought 

 into contact with one another, and a faint shock will also be felt 

 on touching them. 



Arrangements like this are called dry piles, and will continue 

 in action for several years. The discs placed between the 

 pieces of metal are made of some substance that will absorb the 

 moisture from the air, since if they become perfectly dry, all 

 evolution of electricity will cease. The quantity produced by 

 them appears to be exceedingly minute, but it has a considerable 

 amount of tension. 



The most common form of making a pile of this kind is that 

 introduced by Zamboni, and named after him. In the construc- 

 tion of one of these, some sheets of paper are silvered or 

 tinned on one side, and the other side of each is well rubbed 

 with powdered black oxide of manganese. A large number of 

 discs are then cut from these sheets by means of a circular 

 punch, and these are arranged in regular order, one on the 

 top of another. Frequently they are placed inside a glass tube 

 so as to keep them in their places, and brass caps fitted with 

 wires are then fixed to each end of the tube. 



A pile consisting of 1,500 or 2,000 of these discs will have 

 power enough to make the leaves of the electroscope diverge, 

 and condensers may be charged by it. No sparks will, however, 

 bo obtained between the poles, nor will shocks be given by it. 



Attempts have been made by a pile of this kind to solve the 

 question of perpetual motion, and different pieces of apparatus 

 are frequently constructed which will keep in motion for several 

 years, all the motive force of which is derived from a dry 

 pile concealed in the stand. These may often be seen in 

 chemist's and other shop-windows, sometimes in the form of 



