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the spark on the wire, owing to its tendency to escape from fine points into the air, or to flow into the ground through the wood or other substances used as insulators. It was not until a more docile kind of electricity was discovered in the electric current produced by the voltaic battery that the electric telegraph as it exists now became practicable. These early experiments with frictional electricity, however, prepared the way for the later telegraph, by leading up to general laws and suggesting ideas. Moreover, they gave us the lightning-rod, which has proved invaluable in saving life and property from the thunderbolt both on land and sea. When Franklin, in the June of 1752, sent his kite up to the clouds and drew sparks from the end of the wetted string, which in this case served to conduct the atmospheric electricity, he demonstrated the identity of the dreaded lightningflash with the tiny spark from an electric machine. And knowing, as he did, that electricity can be conducted by a metal wire, and that it tends to discharge itself by points, it was an easy matter for him to devise a lightning-rod of copper with one end buried in the ground and the other rearing a fine point above the building to be protected. With this arrangement the electricity of the cloud would discharge itself into the wire by its fine point, and thereby find a harmless path into the ground.
In the year 1800 the celebrated Volta, Professor of Physics at Pavia, invented the chemical generator of electricity which is now known as the voltaic cell, and thereby inaugurated the remarkable development of electrical science which characterises the present century. Volta took plates of two different metals, copper and zinc, and separated them by a layer of acidulated water. On joining the two plates by a wire he found a current of electricity in the wire, and this current became stronger in proportion to the number of pairs of plates which he employed. Each pair of dissimilar metals with its connecting layer of liquid formed a simple element, and a number of these
elements in combination formed a battery. The elementary voltaic cell, as shown in Fig. 4, consists of the plate of zinc % and a plate of copper C, plunged in a vessel of water tinctured
with sulphuric acid and connected outFIG. 4.
side the cell by a wire w w. When the
wire w w is disconnected or broken, no current of electricity is obtained; but when this wire is connected, the voltaic “circuit” is completed, and the current flows in the wire. Chemical action then goes on in the cell, the zinc is oxidised by the oxygen of the water, just as coal is burned in a fire, and the chemical action keeps up the supply of electricity just as the burning coal keeps up the supply of heat. No matter how long the connecting wire is, the current will flow through it, provided it is entire, and hence it is that we have telegraph lines two or three thousand miles long.
The water in this combination supplies the oxygen to burn the zinc, and sulphuric acid is merely added to reduce the resistance of the water to the passage of the electricity through it, for the current starts from the zinc plate where the chemical action takes place, and flows to the copper plate within the cell, then through the wire back to the zinc plate, thus completing its round. The hydrogen set free by the breaking up of the water to yield oxygen to the zinc is given off in the form of gas.
This elementary cell has, of course, been greatly improved upon, and there are now hundreds of different combinations of solids and liquids serving as voltaic batteries. The chief of these are the Daniell battery, the Leclanché battery, the Bunsen, Grove, and Smee batteries. The Daniell battery is the elementary cell we have described, with crystals of blue vitriol, or sulphate of copper, added round the copper plate. This salt is decomposed by the free hydrogen into pure copper and sulphuric acid, so that the hydrogen, instead of being uselessly given off to the air, helps to provide sulphuric acid to reduce the internal resistance of the cell. Moreover, as all the hydrogen is not discharged in the elementary cell, but part clings to the
of the copper.
copper plate, and weakens the power of the cell by an effect called “polarisation, it is highly necessary to get rid of the hydrogen somehow, and there could hardly be a better way than in making it keep down the internal resistance of the cell. Polarisation is due to the fact that the presence of the gas on the copper acts as if the hydrogen had taken the place of the copper in the combination. Now the power of the cell depends on the two plates employed, just as the charge of electricity developed by friction depends on the materials rubbed together, and it does not do to allow an inferior substance like hydrogen to take the place
All cells in which this effect of polarisation," or the polarising of the negative plate, is not well prevented are liable to get feeble during action, and yield an inconstant current. Daniell's battery gives the most constant current of all, and hence is largely used on telegraphs where there are many messages to be sent.
A convenient form employed in America is shown in Fig. 5, where z is the zinc plate, cast in the form of a star, and plunged in sulphuric acid and water ; c is the copper plate, consisting of two bent strips riveted crosswise; and s is the sulphate of copper in crystals at the bottom of the glass cell which contains the whole. The two cells are joined up together “in tension,” that is to say, in such a way as to give the greatest power of sending the current through a long wire. For this the copper plate of each cell is connected (by the wire w') to the zinc plate of the next, and so on through the whole battery, which may number fifty cells or more. The working current is in this case drawn off by the terminal wires w w", and it has a power of overcoming resistance proportional to the number of cells in the battery. This is the manner in which a battery is connected up for telegraphing over long lines; but when the current is to
be used for magnetising purposes, or for electroplating, the battery is joined up“ in quantity,” that is to say, in such a way as to give the largest quantity of electricity, though its actual power of overcoming resistance may be small. This is done by connecting all the zinc plates together and all the copper plates together, thus forming one large zinc and one large copper plate; in short, one aggregated cell.
The Leclanché battery is not very constant, but it has the merit of rapidly recovering its primitive power when allowed to rest a short time. Moreover, it is clean, convenient, and portable, and hence it is very