VOLTAIC ELECrRICITV 



197 



an instrument called the indnctorium, or induction coil, where 

 the effects are magnified to a surprising degree, and give results 

 nost brilliant and startling character. 



nplc form of induction coil is shown at Fig. 86. It con- 

 sists mainly of two separate coils of insulated wire, one within 

 tin- . .t lu-r. The innermost consists of two layers of thick copper 

 wire, which, l>y proper attachments, can be joined up with a 

 .o secondary coil lies outside the primary, and is 



f several layers of very fine silk-covered copper wire, 

 measuring, in large coils, many miles in total length. These coils 

 are tirmly wound upon a hollow core, within which lies a bundle 

 of in.n wires, forming an electro-magnet whenever the current 

 flows through the coil. In front of one end of this core is a vi- 



: armature, the to and fro movements of which cause the 

 cm -rent to be made and broken several times in every second. 

 The object of this is to gather np the transient currents in the 

 secondary coil caused by induction, so as to make them behave 

 somewhat like a continuous current. If this arrangement were 

 not adopted, there would only be a current at the moment of 

 joining the battery to the apparatus, and another current when 

 it was detached. The vibrating armature causes these joins 

 and breaks to occur 

 several times (many 

 hundred if required) in 

 every second, so that 

 we have a constant 



X 



stream of electricity, 

 bat which we know 

 really to be intermit- 

 tent, from the circum- 

 stances under which it 

 is produced. This con- 

 tact-breaker, as it is 

 called, acts in precisely 

 the same manner as 

 the hammer of a 

 1 ' trembler ' ' electric 

 bell ; it is shown in 

 detail at Fig. 87. 

 Here B represents the 

 magnetic core of the 

 coil, c the armature set 

 npon a springing piece 

 of metal A. E and D 

 are adjustable screws, 

 the former causing the 



current to be broken directly A touches it, and the latter 

 restoring the current, so that A is once more attracted towards 

 the core B. 



The phenomena connected with induced electricity can be 

 very well studied by simple apparatus. A battery cell is indis- 

 pensable, and perhaps the best for the purpose is that form 

 known as the Bichromate battery. It is simple and clean in 

 use, requires but one fluid, and gives off no unpleasant fumes. 

 It is generally sold in the form of a bottle containing two plates 

 of carbon and a central plate of zinc. These plates all hang by 

 rods from the ebonite cover of the bottle or jar, but the central 

 one, the zinc, is so arranged that it can be drawn completely np 

 into the neck of the bottle, and out of the exciting solution. 

 Owing to this arrangement, the bottle battery can be kept 

 ready charged for months at a time, the zinc being depressed 

 into the fluid only when the battery is required for use. The 

 charging fluid is made thus : An ounce of bichromate of potash 

 is dissolved in half a pint of boiling water. When the solution 

 has become cold, one ounce of commercial sulphuric acid (oil of 

 vitriol) is added to it. This addition will cause the liquid once 

 more to become hot. When cold it is ready for use. 



Fig. 88 shows an induction coil complete, with a condenser 

 arrangement c, by which the induced or secondary current 

 is increased materially. With such a battery, a galvanometer, 

 and a few pieces of copper wire, some experiments may be per- 

 formed showing clearly how a current circulating in one wire will 

 induce a secondary current in another wire wholly detached from 

 it. First, let one pole of the battery be connected with a wire 

 stretched across two wooden uprights ; let us call this wire A. 

 A second wire, B, is stretched between the same uprights, half 

 an inch below A. This second wire is placed in circuit with a 

 galvanometer. On completing the battery circuit so that the 



current traverses A, the galvanometer needle if deflected for an 

 instant It again more*, but in the reverse direction, when the 

 <-in:uit in broken. Thin experiment proves two important fact* : 

 first, that a current is induced in a wire by the uear presence of 

 a wire through which a current is flowing ; and, secondly, that 

 the current so induced is but transient, and only ocean at the 

 moment when the circuit of the A wire U completed or broken. 

 By fastening a file to one wire loading from the battery (bat in 

 other respects maintaining the condition of affairs j mt described) 

 and making contact by drawing the connecting wire slowly 

 along the roughened surface, we shall have a ready mean* of 

 making and breaking the current alternately, for as the wire i 

 dragged over the projections and depressions of sach a car- 

 face, the current is caused to be intermittent. Under such 

 circumstances, the galvanometer needle will be kept oscillating 

 so long as the wire is dragged over the file. The vibrating 

 contact-breaker attached to the induction coil, or Rhumkorff 

 coil, as it is sometimes called, after its inventor, accomplishesx 

 in a far neater manner the same work. 



Referring once more to Fig. 86, we mast notice that the wires 

 from the battery (not shown in the sketch) are connected with 



the coil on the right- 

 Fig- 86. \~ lf*F hftnd 8iJe - Bat the 



wires from the second- 

 ary coil are carried to 

 the little glass pillars 

 shown on the left-hand 

 side. These pillars are 

 furnished with binding 

 screws, so that any 

 piece of apparatus can 

 be readily joined np to 

 them. 



A very important 

 part of the induction 

 coil is the condenser, 

 which is usually con- 

 tained in the base of 

 the instrument. This 

 consists of fifty or sixty 

 sheets of tinfoil, nearly 

 as large as the base ,- 

 between each sheet of 

 this there is placed 

 a sheet of varnished 

 paper so as to insulate 



them. The pieces of tinfoil are so arranged that one end of the 

 lowest sheet is left out at one end of the pile, the next projects at 

 the other end,and so on : thus they alternately overlap at each end. 

 The free ends are then connected at each end of the coil, and 

 thus the alternate sheets are connected together, half being 

 joined at one end and half at the other. These ends are then 

 connected with the sides of the break, and in some war, which 

 has not yet been clearly explained, the power of the coil ie- 

 greatly augmented thereby. If an arrangement be made by 

 which the condenser can be thrown out of connection when the 

 machine is at work, instead of bright flashes several inches in- 

 length, we shall only obtain thin and faint spark*. 



If, when the coil is in action, wires attached to the terminal? 

 of the secondary coil be brought within a short distance of one 

 another, a stream of sparks passes between them with a loud 

 crackling noise. An induction coil is usually distinguished by 

 the distance which these sparks will leap across, commonly 

 called the length of the spark. Thus a small coil may give a 

 half-'nch spark, and a much larger one a four or six-inch 

 spark. A few years ago, Mr. Spottiswoode, the late President 

 of the Royal Society, had constructed a giant coil, which gave 

 astonishing effects. Supported on two massive pillars of wood, 

 the weight of the mass necessitated a third support midway 

 between them. There are two primary coils which are readily 

 interchangeable. One differs somewhat from the other, and is 

 intended for a special class of experiments. The primary coil, 

 more generally used, contains 660 yards of copper wire. It is 

 44 inches long, and weighs 67 Ibs. The secondary coil contains 

 280 miles of fine wire. Under favourable conditions this giant 

 coil will give a spark 42 inches in length. The contact-breaker 

 is of special construction, and can be made to give 2,500 break* 

 per second. 



