1905.] on a Pertinacious Current. 85 



at the receiving end ; and they have been so appHed by Dr. Muirhead 

 in Kent and elsewhere. For, on our system of wireless telegraphy, 

 there is an aerial Leyden jar to be charged — one capacity area 

 elevated, and another conducting area near the ground, though not 

 usually connected with it, and sparks have to be caused Ijetween 

 these two, as in Hertz's old arrangement. Such a capacity is too 

 great for a small coil to charge at each impulse of its secondary 

 current — but if valves are inserted and a rapid break employed, the 

 impulses of a number of sparks accumulate until the areas are filled 

 to bursting, and then they overflow through the discharging knobs 

 and give the required signal, the whole operation taking place in a 

 fraction, though with a small portable coil and considerable distances 

 to be reached, not a very small fraction, of a second. 



Another use is for the production of X-rays, especially for visual 

 purposes on a fluorescent screen ; because by the reservoir and trap 

 method of excitation an almost steady illumination can be maintained ; 

 thus giving a much brighter effect than can be produced by inter- 

 mittent illumination, even though the property of the retina called 

 ''persistence of impression" does suffice to mask the really inter- 

 mittent character of the light. For such purposes it is best to use a 

 very large coil and a rapid Tjreak. I prefer to use either a revolving 

 mercury break or a Caldwell ; on the whole, I prefer Mr. Caldwell's 

 modification of the Wehnelt or electrolytic break. It consists of a 

 very small aperture or throttle introduced into the path of a 200-vol 

 circuit, containing self-induction as well as the primary of the coil oi 

 coils, the said aperture or throttle being submerged under a conduct- 

 ing liquid, like dilute sulphuric acid, through which the primary 

 current has to pass ; and I make the apertures in a replaceable crockery 

 hemisphere. The current transmitted through liquid in such a minute 

 orifice is excessively intense ; and the heat there developed, it must 

 be supposed, almost instantaneously vaporises the liquid in the aper- 

 ture, and thereby suddenly stops the current ; but the instant the 

 current stops the steam collapses, the broken liquid is reunited, and 

 the current again flows — to be immediately stopped as before. The 

 rapidity with which this action goes on is astonishing — hundreds of 

 times a second — and since there is no mechanism nor anything 

 requiring attention, until the hole wears too large by reason of the 

 \'iolence of the shocks, there can hardly be any automatic break 

 simpler in character than this ; though it is quite likely that other 

 still more efficient plans will be devised. Indeed, already the method 

 just exhibited in London by Mr. Isenthal, employing the reversal 

 of a " Grisson " electrolytic condenser through the primary of a coil, 

 may possibly be a better one. 



At any rate, by some means or other, the primary current is made 

 violently intermittent ; the secondary is then hkewise an intermittent 

 current of vastly increased force, though less in quantity ; and in its 

 path some valves are placed, as many as are needed. By the use of 



