In Figure 68, the properties of Figures 64 and 67

Fig. 68.

are combined, and the red end is thrown from the reader with doubled energy. In Figure 69, the action

is multiplied to any extent. This is the construction of the ordinary galvanometer, and also that of the acting part in the common English speaking-telegraph.

89. Inductive magnetic power of the galvanic current: its action on steel and on iron; formation of transient magnets: registering-telegraphs.

In treating of pure magnetism we have seen that a magnet-pole, which attracts the red end of a magnetized needle, possesses the power also, in the operation of double-touch, of drawing all the red magnetism of an

unmagnetized steel needle to the end nearest to itself, and thereby magnetizing that needle. And in like manner, if a soft iron bar be presented to it, it converts it for the moment into a magnet in a similar state. It is therefore easy to conceive that the galvanic current may be able to produce analogous effects.

The best form of wire for this purpose is a long spiral. In Figure 70 is exhibited a simple spiral: but

Fig. 70.

the wire may be carried round and round so as to form numerous layers, care being taken that the wire is turned round always in the same direction. Such a spiral constitutes a kind of magnet, which, though acting feebly on external objects, is sometimes useful. But its magnetic effect in the interior of the coil is powerful. Conceiving a mass of red magnetism in the interior, the imaginary insect which we have cited, in crawling along the wire from the graphite end, with his face towards the nearest part of the red mass, would in every part of the spiral have his left hand towards the graphite and therefore the attraction of every part of the coil tends to draw the red magnetism towards

the graphite end, and the blue towards the zinc end. (If the direction of the spiral turns had been opposite, the result would have been opposite.)

Now if we insert in this coil a bar of unmagnetized steel, as in Figure 71, the bar is instantly magnetized,

Fig. 71.

and becomes a magnetic needle with its red pole towards the graphite (the direction of the spiral being as shewn in the figure). This process is extensively used for magnetizing compass-needles.

If instead of the bar of steel we insert a bar of soft iron (usually called the 'core'), the bar is magnetized in the same manner as under other inductive force, having its poles in the same position as those of the steel bar just mentioned. But the magnetism is transient, lasting only as long as the galvanic current lasts. If the current be destroyed by interrupting the circuit in any way, as by cutting the wire at any point, or by separating two portions of the wire which are in contact, or by separating the wire either from the zinc plate or from the graphite plate, or by lifting either of the plates out of the acid,—in any of these cases, the iron instantly loses its magnetism. And this property

is exceedingly valuable, because by it we can make and unmake a magnet at a great distance, even several hundred miles, and in any locality, and even in a moving frame.

A convenient and powerful form is that of the horse-shoe magnet, the wires being arranged as in Figure 72. A piece of iron must be provided, to be

Fig. 72.

pulled by the two poles of the magnet. It is in this form that galvanism is commonly employed for the telegraphs in which permanent impressions are made on paper at the distant station.

C

90. Spontaneous terrestrial galvanic currents: investigation of the magnetic effects due to them, and comparison of these magnetic effects with the magnetic disturbances recorded by the self-registering magnetometers.

In the ordinary system of wire-telegraphs, each wire, when not used in the actual work of transmitting galvanic currents, is detached from all galvanic batteries, and is connected at both ends with the earth.

It was soon found that, when the wires are in this state, galvanic currents sometimes pass through them which are sufficiently strong to cause movement of the galvanometer-needle: and (when a battery is placed in the circuit for giving signals) sufficiently strong to pervert the telegraph-signals. And it was at length discovered that those currents, produced by the earth only, occurred at the same times as magnetic storms. In order to investigate the relation between the earthcurrents and the magnetic storms, two wires were established in connexion with the Royal Observatory of Greenwich; one about 10 miles long, terminating at Croydon, the other about 8 miles long, to Dartford: each wire was carefully insulated in every part except at both its extremities, which were plunged in earth, and the two wires passed through two galvanometers, one appropriated to each wire, in the Observatory. Each of these wires might be expected to bring from the earth at one end to the earth at the other end a portion of the galvanism which is flowing through the superficial strata of the earth.

As it was soon found that currents, weak or strong, were almost always perceptible in the movements of the galvanometer, a self-registering apparatus was prepared. To the needle of each galvanometer a small plane mirror was attached, and the light of a lamp shining upon the mirror was by lenses made to converge, to form a spot upon a revolving barrel covered with photographic paper. Thus two registers were obtained similar in general character to those of the