546 



NATURE 



[April 6, 1905 



INTERRUPTERS FOR INDUCTION COILS. 

 TT has been thought that an account of the more im- 

 portant forms of interrupter would not be unwelcome 

 to readers of Nature. 



A rotating air-break interrupter is shown in Fig. i. 

 An accurately balanced brass fiy-wheel, F w, driven 

 by a small motor, is fitted with two insulating segments, 

 I s, let into its periphery. Bearing on the fly-wheel are two 

 copper gauze brushes, b, and B, ; the circuit is interrupted 

 as each brush slips over from the brass to the insulating 



portion of the rim. It is evident that the arcing which 

 occurs at the break necessitates the use of a fire-proof in- 

 sulator. A small piece of slate (s in Fig. i) is fitted imme- 

 diately behind each brass segment, and this takes the spark ; 

 it is easily renewed, the remainder of each insulating 

 segment consisting of vulcanised fibre. 



So far as the writer is aware, this type of interrupter 

 was first described by Wadsworth in 1894, ^nd was used 

 by Prof. Michelson in some Geissler-tube experiments 

 (American Journal of Science, pp. 496-501, December, 1894). 



As might be expected, the suddenness of the break depends 



on the speed of the motor {or frequency of interruption). In 

 Fig. 2 are plotted the results of some experiments bearing 

 on this point. It will be seen that for a given value (root- 

 mean-square) of the primary current, an enormously greater 

 spark-length — especially with the larger currents — is 

 obtained at the higher speed. 



This form of interrupter is not very expensive, and works 

 very satisfactorily so long as the primary current does not 

 exceed about 5 amperes. It shares with the platinum in- 

 terrupter the atlvantage of cleanliness. Renewals and repairs 

 cost very little, as the only parts which are subjected to 



NO. 1849, VOL. 71] 



any considerable wear are the slate distance-pieces ; the 

 rim of the fly-wheel may occasionally require iruing-up. 

 It is important to keep the edges of the brass contact- 

 segments and the surfaces of the slate distance-pieces clean 

 by the occasional application of fine sand-paper. 



In Fig. 3 are shown the essential parts of the mechanism 

 of a ■' double-dipper " interrupter. The double motor- 

 driven crank, c, carries two connecting-rods, c R, each of 

 which is attached to a cross-head, c H. Each cross-head is 

 fixed to the top of a stiff rod, r, which passes between the 

 guide-springs, G s, and through the guide-block, G B. The 

 latter is supported by a strong bracket, B, screwed to the 

 stand supporting the motor. Each reciprocating rod ends 

 in an amalgamated copper wire, c w, which dips into the 

 mercury. It will be readily seen that by the adoption of the 

 two-crank arrangement the frequency is doubled for a given 

 speed as compared with the single-crank interrupter ; for 

 while with the latter there is only a single break per revolu- 

 tion, the former gives two breaks per revolution, one of the 

 contact-rods or " dippers " entering the mer- 

 cury shortly after the other has left it. The 

 mercury cup itself is made adjustable in a 

 vertical direction, and is, as usual, immersed 

 in alcohol. 



The curve marked " double dipper " in 

 Fig. 4 gives the results of a test with this 

 form of interrupter. The frequency of inter- 

 ruption was 22. The results correspond fairly 

 well with those plotted in Fig. 2 for the 

 rotary air-break interrupter at a frequency 

 of 40. 



This type of interrupter is comparatively 

 cheap and simple, and works very steadily. 

 There is no complicated mechanism to get 

 out of order, and only a small quantity of 

 mercury is required (about 2lb.). 



One of the most successful types of rotary 

 interrupter is the mercury jet interrupter. 

 Several varieties of this have been used. One 

 of the best known is shown in Fig. 5. The 

 vertical motor-driven shaft, s, carries a 

 cylinder, c, the lower portion of which is cut 

 up into a number of teeth, t. The shaft s 

 is continued downwards, and passes through 

 the mercury pump casing. The mercury 

 pump is of very simple construction, and is 

 shown in Fig. 5 (6). Inside a flat oval box, 

 which forms the pump casing, are arranged 

 two thick toothed wheels. One of these is 

 mounted on the lower end of the shaft s, 

 which carries the toothed cylinder. Fig. 5 (a), 

 and drives the other. The wheels fit the 

 inside of the casing very closely, and are 

 arranged to rotate as indicated by the arrows 

 in Fig. 5 (b). The mercury imprisoned 

 between the teeth of the wheels and the 

 casing is consequently carried round and 

 forced through the nozzle. The issuing fine 

 jet of mercury, M j — Fig. 5 (a) — is directed p^- , 

 against the rotating teeth, the break taking 

 place at the vertical edge of a tooth. The height of the- 

 nozzle N is adjustable, and by this means the magnitude of 

 the current may be regulated, as by raising the nozzle the 

 jet will be directed against a tooth for a longer period, 

 and the current will attain a larger value before the break 

 takes place. The entire mechanism of this interrupter is 

 contained in a strong cylindrical glass vessel, the lower 

 portion of which contains mercury, in which the pump is 

 immersed, and with which the pump chamber freely com- 

 municates by means of a suction orifice, while above the 

 mercury is the usual alcohol filling the bulk of the vessel. 



If in good working order, the mercury jet interrupter 

 gives excellent results, as may be seen by referring to the 

 curve marked " mercury jot " in Fig. 4, which corresponds 

 to a frequency of interruption =40. A comparison of this 

 curve with that given in Fig. 2 for the rotating air-break' 

 interrupter at once shows the superiority of the jet inter- 

 rupter. The mercury jet interrupter is much more expensive 

 and complicated than the " double-dipper " type, and requires 

 a larger amount of mercury ; but it yields somewhat better 

 results. 



