231 



UNDULATORY FORCES. ELECTRO-MAGNETISM. [THB COIL MACHIXB. 



the secondary wire, only at the moment that contact is 

 made or broken between the ends of tlio primary wire 

 and the poles of the voltaic battory, which U its noiirco 

 of electricity. The following ox]X)riment will introduce 

 the reader to some facts in ooiiuoction with the subject. 



ExprrimtntlO. Wind ulnmt ten yards of No. 10 gauge 

 covered copper wire on a hollow tube of gutta-percha, or 

 paper, which may bo an inch in diameter, and about 

 eight inches long. On this coil wind one of the fine 

 wires above-mentioned, in the same direction, but of a 

 length of thirty or forty foot ; and let the ends of each 

 coil be kept separate, and extended about a foot or 

 eighteen inches from the coil. Two coils will thus bo 

 formed, one on the other the primary one being iasiile, 

 and the secondary, or finer coil, covering it. The two 

 ends of the secondary coil are then to be attached to 

 the terminals of a galvanometer.* If the ends of the 

 primary coil are then placed in contact with the plates of 

 a voltaic battory at the moment they touch, a current 

 will be induced iu the secondary wire, which will be evi- 

 denced by the deflection of the needle of the galvano- 

 meter. This deflection will speedily cease ; but if con- 

 tiu-t between tlio primary coil and the wire be broken, 

 then tlio needle of the galvanometer will again deflect ; 

 and, as often as these processes are repeated, a current 

 of induced electricity will bo produced in the secondary 

 wire, the direction of which will depend on, but be con- 

 trary to, that of the primary one. 



,-imi-nt 11. If a rod of soft iron bo placed inside 

 the hollow cylinder, surrounded by the coils, and the 

 same process be repeated, it will bo found that the 

 deflection of the needle in the galvanometer will bo 

 much greater, and the iron itself will become magnetic. 

 A double phenomena is thus piesentod namely, the 

 induction of a secondary electric current, and also mag- 

 netic effects. 



It will thus be soon that a current of voltaic electricity, 

 when passing by means of a conducting wire, has the 

 power of producing another current in a conductor lying 

 near it ; and this takes place when the two wires are 

 stretched straight, side by side, equally as well as when 

 they ore coiled up in the form we have named. 



There, however, exists a great advantage in the coil 

 form ; which is, that each successive layer oncoil of wire 

 increases the intensity of the current produced. Indeed, 

 by extending the length of tho secondary coil, most 

 astonishing effects of intense electricity can be produced, 

 hi Klmmkorfs coil (which we shall presently describe), a 

 length of two or three miles of wire is used ; and all 

 the phenomena of frictionol electricity can be procured 

 from it. 



IVrhaps tho most convenient mode of illustrating tho 

 various effects of electro-magnetic induction, will ho tliat 

 of explaining the construction, <tc., of the coil machine ; 

 a common form of which is illustrated in the following 

 engraving. 



The apparatus consists essentially of two parts tho 

 Fig. 17 



coils nnd tho contact-breaker. In Fig. 17, n represents 

 tho cylinder on which tho primary and secondary coils 



8n amir, p. 193. 



re wound ; 6 is a bundle of iron wires, placed iu thr 

 centre of tho coil ; and n and p are the two terminals of 

 the secondary coil. The primary generally contains about 

 100 foot of No. 10 gauge covered copper wire; and 

 secondary has often so much as 300 or 500 feet of No. 22 

 or 24 wire. One wire of the primary coil is attached to 

 one of the binding-screws, in which a wire, ir, from the 

 bat teryc is fixed; and the other wires, of both tlio battery 

 and coil, are in connection with the contact-breaker, 

 which next deserves our attention. 



By means of this apparatus, the current proceeding 

 from the battery is suddenly passed to, and cut off from, 

 the primary coJ. This is effected by means of a small 

 electro-magnet, i : over this is placed, at a small distance, 

 a soft iron keeper, which is connected, by means of a 

 steel spring, h, with a brass pillar, g. A wire is fixed 

 under the instrument passing from one of tho binding- 

 screws ; which holds one of the battery wires. Tho other 

 end of the primary coil is attached to the pillar, d ; and 

 from this stretches an arm, e ; at the end of which is a 

 screw terminating in a metallic point, /. The electro- 

 magnet, t, however, forms part of the circuit, by m< 

 of wires arranged beneath tho foot of tho instrument. 

 Now, if a current of electricity bo made to circulate 

 through the coil by the means we have named, the 

 electro-magnet immediately attracts the keeper ]>! 

 over it, and, by drawing it down, removes the spring to 

 which it is attached from being in contact with the 

 metallic point terminating tho screw, /. As the coil nnd 

 batter}' are, electrically speaking, only brought into con- 

 nection when the spring, h, and tho point, /, are in 

 contactit follows, that tho moment they part, tho 

 current is at once cut off from the coil. On this happen- 

 ing, tho electro-magnet, i, loses its magnetism, nnd, 

 therefore, ceases to attract the keeper over it. Tho 

 spring, h, then acts, and again brings itself in contact 

 with the end of tho screw, f. Tho coil is thus again in 

 connection with tho battery, and the electro-magi i< 

 magnetised. The previous conditions are repeated ; and 

 thus tho contact between the battery and tho coil may bo 

 made and broken with great rapidity. 



As we have already stated, a secondary current is 

 induced in the fine coil each time when contact is mode 

 and broken between tho primary coil and tho battery ; 

 and as this is easily done oOO times per minute, as ninny 

 induced currents are simultaneously produced, and a 

 torrent of electricity afforded. There is, however, a 

 groat difference between tlio character of the two 

 currents. Tliat of the primary, which is derived from 

 the battery, has all the characteristics of the voltaic- 

 current; whilst that afforded by the secondary coil 

 presents effects precisely similar, in many respects, to the 

 phenomena of frictiouol electricity ; and this identity is 

 developed just in proportion to the length of tho coil 

 employed all other circumstances remaining tho same. 



In our engraving, tho terminals of tho fina coil are 

 attached to two handles ; and if these be grasped by the 

 hands, previously moistened with suit and water, 

 a most powerful shock may be obtained. If the 

 secondary coil bo very long, and tho batten 

 tached to tho primary bo in good action, the effects 

 on tho muscles of tho arm ore so powerful, as to 

 make it impossible for tho person receiving tho 

 shock to leave hold of tho handles ; and tho pain 

 produced is most excruciating. In fact, consider- 

 able caution should bo observed in tlus respect, 

 vsi>ccially with young persons and females, as 

 serious consequences may result from inadver- 

 tence. It is better to place the end wires of tho 

 secondary coil in two bnxins of water, into which 

 persons desirous of receiving a shock can ininicrxu 

 * tho hands; and from which they, generally 

 speaking, far more quickly withdraw them. Two 

 or inoro persons can jointly receive the shock by 

 joining hands together. The effect, however, 

 rapidly diminishes as tho number is increased; 

 and varies very much according to susceptibility of each 

 individual. 

 Tliis form of machine is largely used for tho purpose 



