THE ELECTRIC SPARK, ETC.] UNDULATORY F O R C ES. ELECTRICITY. 



173 



that pressure the results were uncertain ; and, indeed, 

 no further benefit could be observed in that respect. 



It must, however, be borne in mind, that the steam 

 was necessarily shut off and thrown on suddenly ; and, 

 of course, such tends to increase the evils arising from 

 "priming," in direct proportion to the pressure employed. 

 Full allowance must therefore be made for errors 

 which may thus arise in computing the effect of an 

 increase of pressure. By careful watching, we arrived 

 at the opinion whether correct or not that an excess of 

 pressure beyond ninety pounds on the square inch, did 

 not produce any advantageous results. 



We have previously remarked, that the length and 

 intensity of spark were modified by shifting the box 

 of metal points nearer to, or further from, the steam 

 issuing from the jets. The effect of these points is 

 simply that of induction a term which we shall have 

 to explain more fully hereafter. The closer the box of 

 points is to the jet of steam, the longer and thinner is 

 j the spark taken from the boiler. When the box is re- 

 moved to its greatest distance from the jets, then the 

 spark assumes its maximum of density and minimum 

 ! of length ; and in this state is most fitted to charge the 

 Leyden jar, and to produce other effects of what we 

 shall venture to call "condensed" electricity. 



A great loss is, of course, sustained by the ascent of 

 hot air from the furnace, which forms the cellular tube 

 of the boiler. This is effected by means of a chimney, 

 which is divided into two parts; one of which is attached 

 to the boiler, and the other is so arranged that it can 

 slide, telescope fashion, into that rising from the boiler. 

 The fuel which is ordinarily used is coke, which has 

 the advantage of producing no smoke, and an intense 

 ; and the furnace is situated in the centre of the 

 r, in an internal flue. Safety-valves and a water- 

 gauge are attached. 



The electrical effects produced by this powerful ar- 

 rangement are of the most astonishing kind. The 

 length of the spark is sometimes nearly two feet, de- 

 pending on the state of the weather ; and during an east 

 wind and intense frost, when the atmosphere is almost 

 destitute of moisture, the maximum effects are produced. 



The spark may be obtained from any portion of the 

 boiler, and is received by means of a large brass ball at- 

 tached to a copper rod. This is connected, by means of 

 wires, with the earth or drain beneath the boiler. A 

 person taking a spark from the boiler, by means of the 

 rod, >tc. , receives no shock, unless his shoes contain 

 iron nails, when the effect is painful. Few persons 

 would repeat a trial of receiving a spark of two inches 

 in length ; and a strong man, who accidentally received 

 one when the steam was issuing at a pressure of ninety 

 pounds, was immediately prostrated, and for some time 

 remained insensible. 



In damp weather, it is essential that the glass legs, 

 or supports, be continually wiped between the intervals 

 of the experiments, with dry cloths. This precaution we 

 have already alluded to as necessary with all kinds of 

 electrical apparatus. 



Hydro-electric machines, of all rizes less than that 

 we have described, have been constructed. They are, 

 however, dangerous instruments in the hands of in- 

 I cautious persons. The brilliant effects they produce 

 I have the tendency of inducing a prolongation of the 

 1 experiments, until the water in the boiler may be re- 

 duced below its proper level ; and, in such cases, an 

 explosion is imminent. 



Having described a variety of forms of electrical 

 machines, we shall proceed to investigate tho numerous 

 phenomena which may be illustrated by them. As the 

 whole of the experiments we shall introduce are such as 

 we have repeatedly tried ourselves, the reader will have 

 no difficulty in producing the results to which reference 

 will be made, if he follow the directions we shall 

 give him. 



THE ELECTRIC SPARK AND BRUSH. 

 WB shall now enter into some descriptions of the dif- 



ferent appearances which the electric light presents in 

 the form of a spark or brush, as produced by electrical 

 machines of any kind that we have already referred to. 

 When a metallic wire is placed in contact with the con- 

 ductor of the machine, all the electrical effects pass off 

 silently, because the wire has sufficient power to convey 

 them away being what is called a good conductor. If, 

 however, an interval of air be allowed between the 

 wire and the conductor, the charge produced by the 

 working of the machine will not expend itself noise- 

 lessly ; but owing to the resistance which the air affords 

 to the passage of the electricity, a brilliant spark is 

 produced, the size of which depends on the power of the 

 machine from which it is obtained : a loud snapping 

 sound is also afforded, which increases in intensity ac- 

 cording to the quantity of electricity set free. From a 

 small machine, having a plate of a diameter of twelve 

 inches, a spark of one inch in length may be obtained ; 

 from one of three feet in diameter, a spark of six 

 inches is produced ; and the Large plate machine of 

 seven feet diameter, and the hydro-electric, already 

 described, afford sparks varying from fifteen to thirty 

 inches in length, depending on the dryness of the sur- 

 rounding air. 



When a large quantity of electricity is in motion, 

 the spark has a deep-blue colour, and a jagged shape, 

 exactly like a flash of lightning ; which it, indeed, really 

 is in miniature. The colour varies according to the 

 medium in which the spark is produced, and also from 

 the substances from which it passes. Some beautiful 

 experiments may be tried by means of an air-pump ; 

 and a convenient arrangement is illustrated in the 

 following engraving. 



Fig. G. 



a represent* the conductor of the machine ; 6 U a wire connecting ttot 

 conductor with the rod c ; e t m rod which fits air-tight in a brass colls r. 

 d ; 4 formi the upper covering of the glan receiver, e ; / is the othci 

 extremity of the brans rod, between which and g the discharge takes 

 place ; A u the cylinder of an air-pump, of which i U the handle. 



With this apparatus the following experiments may 

 be tried : 



ExjKriment 5. Having attached the conductor a 

 to c, draw the rod c / so as to leave a space of six or 

 eight inches between its end /, and g, and exhaust the 

 air from the receiver so as to produce a good vacuum ; 

 connecting the air-pump, by means of a wire, with a gas 

 or water-pipe ; then commence to work the machine. 

 A spark will pass between / and g, of a colour which 

 varies according to extent of vacuum in the receiver, e. 

 If that be good, a fine purple tint is perceptible : if 

 the vacuum be imperfect, the spark, instead of pre- 

 senting a continuous discharge, will be intermittent, and 

 of a red colour, attended with sudden star-like flashes 

 and a snapping noise. When the vacuum is good, and 

 the machine of large size, this forms one of the most 

 beautiful experiments in physical science. The spark 

 from the hydro-electric machine, when passed through 

 a tube five feet long and three inches in diameter, pro- 

 duced a magnificent appearance of purple flame, which 

 completely filled the tube so long as the vacuum was 

 maintained. On diminishing this by a gradual admis- 

 sion of air, the continuity of the spark ceased, and in- 

 termittent discharges, of a blue colour, were produced. 

 The engraving (Fig. G) gives an idea of this appear- 

 ance, so far as shape is concerned, and as it may be 

 obtained by means of a small-sized air-pump. 



i'.rperiment 6. After the air has been exhausted 

 from the receiver, admit, by means of the small screws, a 



