CHAMBERS'S INFORMATION FOR THE PEOPLE. 



a steady flow of ink in a fine stream, just as in 

 the case of the ' electric pail.' 



As a proof of the skill and genius brought to 

 bear on the Atlantic telegraph, and of the delicacy 

 of Sir W. Thomson's instruments, it may be 

 mentioned, that a tiny cell, consisting of a copper 

 percussion-cap and a small piece of zinc, has 

 been found sufficient to send a message across 

 the Atlantic. In spite of all the difficulties of the 

 case, signalling is now done as rapidly through 

 it as on any land-line of the same length. Public 

 messages are sent at the rate of eight to fourteen 

 words a minute ; but when the clerks communi- 

 cate with each other, as high a speed as twenty 

 words a minute is attained. 



ELECTRICITY OF CURRENT INDUCTION. 



Faraday discovered, in 1832, that current elec- ! 

 tricity may be developed by induction, in a manner 

 somewhat analogous to that in which statical elec- j 

 tricity is induced. If a wire, through which a 

 current passes, be brought near to another whose 

 ends are fixed in the binding-screws of a galvan- 

 ometer, there is an instant deflection of the needle. 

 It soon falls back, however, to its original position, : 

 and not the slightest effect is seen so long as the ' 

 current continues steadily to flow. On withdrawing I 

 the wire, or on stopping the current, we again have 

 a deflection of the needle, but this time to the other 

 side. As often as we join and break the current, 

 we have these momentary flows of electricity in- 

 duced in the galvanometer wire, which will set the 

 needle a-swinging from side to side. 



In general, then, it may be stated that the pas- 

 sage of a current in a conducting circuit produces 

 a sympathetic electric state in a neighbouring j 

 circuit ; so that any change of condition of the 

 first is accompanied with a corresponding electric 

 throb of the second. The effect on the galvan- 

 ometer circuit will be greater the more rapidly we 

 change the state of the inducing electricity, and 

 the greater the extent of the second circuit influ- 

 enced by the current. 



We must note the directions of the induced 

 currents. By the approach or commencement of 

 an inducing or primary current, a wave passes 

 in an opposite direction through the induced or 

 secondary circuit ; and by the break or with- 

 drawal of the primary current, a wave passes 

 along the secondary circuit in the same direc- 

 tion as that of the primary. Or, shortly, at make 

 and break of primary circuit, an inverse and 

 a direct secondary current are respectively in- 

 duced. There is thus a great difference between 

 current and statical induction. So long as the 

 prime conductor of a machine is charged, it has a 

 constant inducing influence on all bodies near it. 

 But it is only a change of tension in current elec- 

 tricity which has any inducing power, and the 

 power is momentary as the change. 



We shall now describe the method of producing 

 these currents adopted in practice. The induction 

 coil is the apparatus employed for this purpose, 

 and the best form of it is that known as Ruhm- 

 korff's coil. 



Essentially, it consists of two bobbins or coils 

 of insulated copper-wire, an inner and an outer. 

 The inner coil is of thick wire, and through it 

 passes the primary or battery current. It is called 

 the primary coil. The outer is of fine wire, and 



282 



in it is induced the secondary current, at make 

 and break of the primary circuit. It is called the 

 secondary coil. Sometimes the inner coil is made 

 so that it can be withdrawn from the secondary at 

 pleasure. 



The object of this coil-arrangement of the wires 

 is to make each turn of the primary or inducing 

 wire act not only on the turn of the secondary 

 next it, but also on all the turns near it. A vastly 

 greater effect is thereby obtained than if the two 

 wires were simply laid side by side in a straight 

 line. For this reason, too that the strength of 

 the induced currents depends on the extent of 

 secondary wire under the influence of the primary 

 very fine wire is used, to give as great a length 

 of it as possible. Induced currents are of high 

 tension compared with battery currents, and the 

 fineness of the wire is no obstacle to the passage 

 of the currents. Only, the finer the wire, and the 

 higher the tension of the secondary currents, the 

 more careful must be the insulation of the differ- 

 ent turns and layers. Everything depends on the 

 proper insulation of the secondary coil. 



We have said that the wire of the primary coil 

 is thick. This is to offer little resistance to the 

 battery current, and to give the maximum of mag- 

 netic effect. For the inducing power of the 

 primary increases with its magnetic power. In 

 proof of this, it is found that the addition of a soft 

 iron core in the primary coil increases enormously 

 the strength of the induced currents ; and we 

 know it heightens the magnetism of the inducing 

 coil. In further proof of this fact, and in singular 

 confirmation of Ampere's theory of magnetism, a 

 permanent bar-magnet may be put for the primary 

 coil. A current in one direction, or the reverse, is- 

 induced when it is put in or taken out of the 

 secondary coil. 



Such are the general features of the induction 

 coil. Fig. 40 will give an idea of its appearance. 

 The primary coil is inside and out of view, and 



Fig. 40. 



the secondary, W, alone is seen. The ends of the 

 secondary wire are soldered to two brass heads, 

 P, P', insulated on glass pillars, and carrying 

 pointed brass rods, capable of universal motion. 

 It is between these brass points that the induced 

 current leaps across. The two battery wires are 

 attached to the two binding-screws, p and n. 

 Before reaching the coil, the current has to pass 

 through, first, the commutator, C, and, second, a 

 self-acting break not seen in the figure. With the 

 commutator we can turn the current off or on, or 

 reverse it at pleasure, by giving it a quarter or a 

 half turn. 



The automatic break interrupts the current with 

 extreme rapidity, and so gives rise to an incessant 

 stream of induced currents. 



