UNDULATORY FORCES. ELECTRO-MAGNETISM. [HPAUK IN 



Fig. 19. 



Experiment 15. Employ the apparatus used for send- 

 ing the electric spark in a vacuum, described under the 

 section of Electricity* attaching the outer wire of the 

 secondary coil to the ring of the rod, and the inner wire 

 to the metal plate of the pump. A rich purple-coloured 

 flume will poos, which, however, will depend for iU 

 length and beauty on the degree of exhaustion of the 

 air in the receiver. If a good vacuum be maintained, 

 then the flame will be of a moat delicate purple or violet 

 tint. If air be slowly admitted, the colour of the 

 llnmo becomes gradually rodder, and, eventually, a red 

 wire-like spark will be produced. 



\\V have repeatedly tried, at the same time, this ex- 



E'mcnt with the Rhumkorf, and a similar one with the 

 o hydro-electric machine at the London Polytechnic ; 

 the vacuum being equal, the effect in each receiver 

 was precisely the same. 



In 1855 and 1856, being engaged in a series of ex- 

 periments with Rhumkorf s arrangement, we found the 

 barometer tube of great service, as 

 affording a complete vacuum ; but, 

 for this purpose, a platiua wire 

 must be soldered into the top of 

 the tube before it is filled with mer- 

 cury. This arrangement has already 

 been referred to :t when it is used 

 with the Rhumkorf, the outer wire 

 of the secondary coil is to be attached, 

 by means of a lino wire, to d, the 

 piatina wire ; and the inner wire of 

 the coil is to be dipped into the 

 mercury of the cistern 6. On passing 

 the secondary current through the 

 vacuum c, a beautiful lavender- 

 coloured flame is produced. 



Experiment 1C. Amongst other 

 results obtained, were many curi- 

 ous appearances, produced by passing 

 sr..all portions of the vapour of ether, 

 bisulphide of carbon, <irc., into the 

 vacuum of either the air-pump or 

 the barometrical arrangement. This 

 e ^y ***! * the former by 

 mercury; , the cu- loosening the screw which admits 

 tnof that metal; c, air jj,to the receiver, and holding 



the vacuum; a, the . . vxii u. i 



platinum wire; and against it a little cotton wool moist- 

 t, a ring-stand to eiied with the liquid. The air must 

 %*$. "" " t* a ain "Busied ; the current may 

 thm be passed, when the diffused 

 vapour left in the receiver will at once affect the 

 normal colours of the discharge. Bisulphide of carbon 

 is generally at oncn decomposed a bright flash of flame 

 being produced at first ; after which sulphur is deposited 

 on the surface of the receiver. Ether, naphtha, ike., produce 

 a flame of a greyish-lavender tint. Some very interest- 

 ing effects are also produced by passing the spark through 



. il atmospheres of hydrogen, carbonic acid, ic. 

 In trying these experiments, it will be noticed, that 

 one pole presents a beautiful blue spangle |of flame ; 

 whilst the other is of a red hue the position of these 

 appearances being altered on a reversal of the primary 

 current being effected by means of the commutator ; and 

 we may ado, that the appearances undergo constant 

 variation OB the vacuum of the receiver varies. \\ 

 have met with few air-pumps which have been so well con- 

 structed as to permit of maintaining a sufficiently good 

 vacuum for any length of time ; indeed, to a practised 

 eye, the colour of the discharge in vacuo we have been 

 describing, is as good an indicator of the extent of ex- 

 liaustion as is the barometric column. 



A very pretty mode of exhibiting these results, is that 

 of placing inside the receiver a small glass vessel, the 

 inside of which has been partly lined witli tinfoil All 

 the arrangements are to bo carried out in a similar 

 manner to those already described, with the exception 

 that the couducting-rod fitting into the top of the re- 

 ceiver, U forced down so that its lower part may rest 

 inside of the small coated jar, and with its end touching 



Btt mil, p. ITS ; Fig. 6. f Sec anlt, p. 171, 



z=k 



, re om ir 



inside of the coil ; c, the air-pump ; 

 d d , a brass rod fitting air-ti^lit in 

 the receiver, mid re^tin^on the coat- 

 ing of the jar r ; f, the *emi*coated 

 jar, out of which the cascade of flame 

 proceeds to the plate of the pump. 



the coating. (See Fig. 20). A spark passed in such a 

 manner, produces a fine cascade of flame, which presents 

 a singularly beautiful ap- n f . 20. 



pearonce. We are in- 

 debted to Mr. Gossiot for 

 this interesting mode of 

 carrying out too exi>eri- 

 ment Tables, and vessels 

 of various shapes and ma- 

 terials, are sold by the in- 

 strument-makers, for the 

 purpose of exhibiting these 

 luminous effects in a va- 

 riety of ways. 



A Leyden jar may bo 

 easily charged and dis- 

 charged, by attaching it 



?>l,,, !._* !?_ it,:, a Is a wire from the outside of the 

 to a Rhumkorf. For this u^aA^y C oil ; *, a wire from the 



purpose, a stout wire 

 should be twisted round 

 the outside coating of a 

 jar ; and one of its ends 

 is to be bent up so as to 

 be nearly in contact with the knob of the inside of 

 the jar. The outside of the jar is to bo attached \>y 

 a wire to the inside of the coil, and the outside of 

 the coil to the rod rising from the inside of the jar. (See 

 Fig. 21). The latter is to be insulated by placing it on a 

 sheet of glass, gutta- 

 percha, or other non- 

 conducting material. 

 On passing the bat- 

 tery current through 

 the primary coil, a 

 series of brilliant dis- 

 charges will take place 

 from the secondary 

 coil, between the knob 

 of the jar and the 

 external wire : these 

 ore attended with a 

 loud snapping noise, "itheT.eyclenjar; t.ttiewirrnirrounilin; 

 j 41' it ; c, a wire reaching nearly to the knob 



and are repeated in- of lhc jar, but docs not touch it ; d is 



CCSSautly. the interval between which the spark 



Experiment IT If P"""'. ' "" "'" attached to the inside 



, ,7 of th<! K-condary coil; /, the wire at 



a cord, or a few folds Of tached to the outside of the coil; p. a 

 paper, be interposed gutta-percha plate, on which the jar 

 between the knob and 



the wire, they will bo instantly perforated ; and a thin 

 sheet may be easily inflamed. A little naphtha, ether, 

 or spirits of wine, if poured on to the knob of the jar, 

 may also be readily ignited. 



Many of the leading experiments of frictional elec- 

 tricity may be repeated by means of the Rhumkorf 

 coil ; amongst which are the following. 



Experiment 18. Suspend two pith balls, by means of a 

 silk thread, from a glass tube, and attach to the top of 

 the thread the wire from the outer end of the coil : the 

 pith balls will at once diverge, owing to the charge of 

 intense electricity just as may bo observed when a 

 pair of pith bolls are hung on the conductor of the 

 ordinary electrical machine. 



ivriment 19. Hold a pith ball, suspended from a 

 well-dried glass tube by means of a silk thread, near to 

 the outside of the coil, in any port of its surface, 

 ball will bo attracted to it, as if it were a charged con- 

 ductor; and if a metallic ball, attached to a chain or 

 wire, be also presented, sparks may be readily obtained 

 from the coil. In trying this experiment, some risk 

 occurs of injuring the silk covering of the wire of 

 secondary coil. In connection with the experiment, wo 

 may mention, that, in very dry weather, sparks often 

 dart from the outside of the secondary coil, to the wires in 

 different parts of the instrument ; and a powerful shock 

 can bo obtained from that part as easily as from the 

 terminal wires or poles. The reader will ']>' -nvhe, that 

 in this fact exists another analogy betw cm the electricity 

 produced by a Rhumkorf's coil and that by friction. 



Most of the other class, experiments exhibited in 



