248 



NA TURE 



[January 15, 7903 



THE HEWITT MERCURY LAMP AND 

 STATIC CONVERTER. 



ONE of those happy discoveries which at once and 

 unexpectedly supply the solution of a difficult or 

 hopeless problem was brought to the notice of a limited 

 number of railway and of scientific men last Friday 

 evening by Mr. George Westinghouse. The company 

 were invited to meet Mr. Westinghouse at the Westing- 

 house Company's office in Norfolk Street, Strand, to 

 see two of the inventions of Mr. Peter Cooper Hewitt, of 

 New York, and to meet again at Claridge's Hotel, after an 

 hour, in circumstances that would enable them more easily 

 to contemplate the full beauty of what they had seen. 



The mercury vapour lamp consists of a long vacuum 

 tube, perhaps a yard long and an inch in diameter, but 

 of dimensions depending on the current and potential 

 available and the light required, with an electrode at 

 each end, but at the lower end, which is the negative 

 pole, the tube is blown out into a bulb, which contains 

 a quantity of mercury. When the ordinary voltage of 

 a house supply is applied to the terminals, nothing 

 happens at all, as it is not sufficient to break across the 

 long, vacuous gap. If, however, a single spark from an 

 induction coil is sent from one terminal to the other, the 

 current from the house mains is immediately started and 

 continues to pass indefinitely, producing a brilliant light 

 absolutely without flicker, but of a ghastly hue. It is a 

 splendid and economical light, requiring, according to 

 the information furnished, only half a watt, or under 

 favourable conditions only one-third of a watt, to the 

 spherical candle-power, whereas ordinary incandescent 

 lamps require about four watts to the candle-power. It 

 is a beautiful light to work by so long as colour is not a 

 point — for instance, for engineers' drawing or for lathe 

 and machine-shop work — as the extent of the luminous 

 surface does away with the glare and the contrast of 

 sharp shadows cast by lamps of small surface. It is, no 

 doubt, valuable for lots of things, and there is nothing to 

 wear out. But the colour ! It is not like the sodium 

 light, practically monochromatic, so that all colour differ- 

 ence is abolished and everything becomes black or 

 yellow or something between the two. That is merely 

 hideous. Here, however, there is plenty of colour. 

 The spectium shows two bright orange lines, a green 

 line, a pale greenish-blue line and a dazzling blue line, 

 but no red. The result is that flowers and coloured 

 articles appear wonderfully coloured, but not with 

 their own natural colours, and what any pigment will 

 look like no one can tell, nor can the faintest 

 idea be formed of what the colour of anything seen by 

 the light of the Hewitt lamp really is. The light plays 

 such pranks with colour that the colour sense seems to 

 have gone crazy. One red thing will appear blue, 

 another black, one blue thing blue, another brown, but 

 the skin becomes ghastly. If anyone sees himself in 

 a glass, it is difficult for him not to form a sort of opinion 

 that he is killed and drowned and dead as well. These 

 effects the Westinghouse people believe may somewhat 

 interfere with the success of the lamp as a domestic 

 luminary. But even here there are possibilities. A 

 wisp of silk dyed with a particular crimson dye appears 

 to have its colour enhanced. It shines with a glorious 

 luminosity among its surroundings, on which not a trace 

 of a rosy tone can be discovered. This is a true 

 fluorescence. If a spectroscope be turned on the lamp 

 or any ordinary thing lighted by it, the red end of the 

 spectrum is absent, but when this particular dye is 

 brought up, the whole of the red end flashes out, and 

 other things may be seen more as they are. A striking 

 experiment is to look at the lamp through ruby glass, 

 through which hardly any light can be detected, and 

 then to bring up the dyed silk, which immediately 

 appears to create its own light and shine brilliantly. 



NO. 1733, VOL. 67] 



Enough has now been said to give an idea of the Hewitt 

 lamp, which is found to have the remarkable property, 

 one not unknown as a vacuum phenomenon, of only 

 allowing a current to pass in one direction, that being 

 with the mercury as a negative pole. If it is attempted 

 to send a common alternating current through a Hewitt 

 lamp, it may be started by a preliminary spark, but at 

 the first reversal it goes out, and so it has to be started 

 perhaps a hundred times a second to keep it going. If, 

 however, the three ends of a star-wound triphase trans- 

 former or generator are connected with three electrodes 

 near the top of a globe and the common centre is connected, 

 with the mercury pole at the bottom, then, as before, 

 nothing will happen until a starting spark has been sent 

 across the globe, for which purpose a fifth electrode is- 

 placed at the top ; then at once the triphase current 

 starts running round from electrode to electrode, and 

 always going to the mercury below, and each current 

 being still alive when the next is ready to start, they 

 keep each other going and a single direct current 

 leaves the mercury electrode. By this simple means, it 

 is possible to rectify a current of even iooo volts, 

 subject, however, to a constant loss of 14 volts in 

 the bulb, and this whatever the voltage. As the con- 

 trivance will work with anything between 100 and 

 1000 volts, and at present up to 100 amperes, it will 

 be evident that if further experience bears out the in- 

 formation so far available, the present methods of con- 

 version depending on the use of rotary converters and 

 motor generators will be at an end, and the labours and 

 ingenuity of Mr. Pollak and others with the aluminium 

 cell latgely superseded. With the higher voltage, the 

 economy is unapproached by other methods, the loss 

 being only 14 percent., which appears as heat in the 

 bulb, C. V. B 



THE VIBRATIONS OF GUN BARRELS. 



A SERIES of experiments has been conducted by 

 ■^*- Messrs. C. Cranz and K. R. Koch for the purpose 

 of obtaining information respecting the character of the 

 vibrations set up in the barrel when a gun is fired. It 

 is a matter of experience that when a cylindrical rod is 

 struck by an approximately axial blow, the particles of 

 the rod, instead of vibrating in straight lines, perform in 

 general elliptic vibrations the axes of which vary in 

 direction at different points, and it was one of the objects 

 of the investigation to ascertain how far a gun barrel 

 behaved in the same manner. 



For this purpose, a number of military rifles supplied 

 by the firm of Mauser were furnished with projecting 

 wires the motions of the shadows of which, thrown on a 

 screen by a powerful lens, were recorded by photography, 

 a tuning-fork similarly projected affording a standard of 

 comparison from which the period of vibration could be 

 measured. 



The rifles were either fixed in a support of cork or 

 held in the position usually adopted by marksmen, under 

 conditions closely resembling those existing in actual 

 rifle practice. By means of an electric spark, a mark 

 was recorded on the photographic plate indicating the 

 exact instant at which the projectile left the barrel. 



An example of the diagrams obtained is shown by 

 Fig. 1 for a rifle fixed in cork and by Fig. 2 for free 

 firing. It will be observed that under the latter condi- 

 tions a dark shadow is in general produced by the recoil of 

 the rifle, and it is only possible to study such parts of the 

 vibration curves as are not blotted out by this shadow. 



The experiments show that the vibrations are in 

 general, as predicted, elliptic in character, each vibrating 

 particle describing a small ellipse instead of a straight 

 line. The vibrations are generally similar to those of an 

 elastic rod fixed at one end, and consist of a fundamental 

 tone and overtones, of which as many as three have been 



