September 24, 1891] 



NA rURE 



495 



blue lamps behind any side wing, top drop, or set piece, 

 can be separately turned on, or all can be turned on and 

 the brightness of the lamps of any one colour varied 

 independently of the brightness of the remainder. 



A bell tinkles, and the curtain rises, showing a pretty 

 set scene of a Swiss village with mountains in the back- 

 ground. It is late in the afternoon. The attendant slowly 

 revolves one of the resistance handles -the daylight 

 wanes, the shadows grow long, the sun sets, and the 

 snowy peaks of the mountains are ruddy with the Alpine 

 glow. The effect is so lifelike and so beautiful that a spon 

 taneous gasp of admiration is forced from the audience. 



Then the stage grows gradually dark, lights are seen at 

 the cottage windows, but the night is stormy, for the 

 attendant now works the handles rapidly, as does the 

 organist the stops when performing one of Bach's fugues : 

 lightning plays on the hills, now a blinding flash lights 

 up the road, the houses, and the waterfall ; but the flashes 

 grow less vivid, and one sees, or thinks one sees, the 

 storm blowing away over the mountain tops. Presently 

 the moon rises, the audience feel the quiet of the bright 

 moonlight night, then the dawn, and finally the sunshine 

 bathes the scene with light. 



Since the opening of the Exhibition many theatrical 

 managers, we were told, had ordered complete sets of 

 this electric apparatus ; and no wonder, for on it can be 

 played a symphony in the music of colour. 



We next went to see Messrs. Siemens and Halske's 

 " dancing flames," the seats at this show being also 

 well filled with a twopence-halfpenny paying audience. 

 First, Koenig's manometric flames were described and 

 shown in action ; then Dr. Froelich's method of working 

 them from a distance, the elastic membrane of the little 

 gas-bag being pushed in and out, not directly by the air 

 puffs, but by the motion of the ferrotyped iron disc of a 

 telephone, the current through which was varied by 

 speaking to a microphone. Next were shown some ex- 

 periments, extremely interesting to the electrician, for 

 illustrating graphically how self-induction, mutual induc- 

 tion, capacity, «&;c., attected the current produced by an 

 alternate current dynamo. 



We presume that the considerable number of people 

 who, having paid for their entrance to the Electrical 

 Exhibition, are willing to form group after group and pay 

 an extra fivepence at the many performances that are 

 given daily of these two shows by Messrs. Siemens and ! 

 Halske, are not wholly ignorant of what they are paying | 

 to see. Probably, therefore, the continued attraction i 

 which such shows have for audiences drawn from the [ 

 people is only another proof of the fact that science, and 

 a love of science, have permeated to a much lower ' 

 stratum of the nation in Germany than in England. | 



Numerous must be the Germans not much above the ! 

 level of the sightseers at a village fair who have already 

 listened to the explanation of Dr. Froelich's method for ; 

 exhibiting these alternate current phenomena, and yet 

 the method is novel to the majority of the English scien- 

 tific visitors. For it was only some three months ago, 

 when Prof. Perry showed his new steam-engine indicator 

 to the Physical Society of London, that the President 

 suggested how he thought it possible that that in- 

 strument might be converted into an oscillating tele- 

 phone with a mirror on its iron disc, and used for pro- 

 jecting on a screen the current curve of an alternate 

 current dynamo. But nobody at the meeting was appa- 

 rently aware that Dr. Froelich had been employing a 

 telephone with a mirror on its disc for this very object — 

 such is the resistance to the spread of ideas introduced 

 by difference of language. 



The apparatus employed by Dr. Froelich is as follows : 

 — A large telephone iron disc has a small piece of 

 looking-glass stuck on it eccentrically, and at the back is 

 a horse-shoe permanent magnet, the soft iron pole-pieces 

 of which are wound with a coil carrying the current pro- 



NO. 1 T43. VOL. 44] 



duced by an alternate current dynamo. The iron disc is 

 therefore pulled more or less by the magnet, depending 

 on the strength and direction of the current passing round 

 its poles. A beam of light from an electric lamp is 

 reflected from this mirror on to a screen, and as the alter- 

 nating current flows round the magnet a vertical line of 

 light is formed on the screen, the position of the' 

 spot of light on this line being at any moment a 

 measure of the strength and direction of the current 

 produced by the machine. At least, this will be the case 

 if the natural period of vibration of the telephone plate 

 be very small or very large compared with the periodic 

 time of the current— a condition we presume Dr. Froelich 

 has attended to. 



To produce a motion of the spot of light ai right 

 angles to the former line, Dr. Froelich does not cause 

 the telephone to be moved backwards and forwards with 

 an oscillatory motion, by the rotation of the dynamo 

 armature, as suggested at the Physical Society of 

 London ; but before the beam of light reaches the screen, 

 he causes it to suffer a second reflection from one of a 

 series of small plane vertical mirrors, arranged around the 

 surface of a cylinder parallel to its axis. By suitable 

 worm-gearing, the quick rotation of the dynamo causes a 

 somewhat slow rotation of this cylinder, but quick enough 

 to produce an apparently continuous horizontal beam of 

 light along the screen if there be no current flowing — that 

 is, if the mirror on the telephone plate be at rest. 

 Hence, the combination of the vertical and horizontal 

 motions of the beam produces a curve which shows the 

 shape of the current-wave extending over some four or 

 five periods. 



The effect of adding self-induction or mutual induction 

 or capacity to the circuit is instantly seen by the change 

 in the shape of the current-curve on the screen, and the 

 change of phase is also evident from the shifting of the 

 whole series of waves sideways. The comparison be- 

 tween the current waves in the primary and secondary 

 circuits of a transformer is also very prettily illustrated. 



This lecture concluded with an exhibition of an ap- 

 paratus that has been constructed for Dr. Froelich for 

 the examination of compound sounds. On a shaft, 

 turning at a uniform velocity, are eight little alternate 

 current dynamos, and by pressing down a piano key, 

 which closes the circuit of the particular dynamo, a 

 current is sent round the soft iron pole-pieces of the 

 horse-shoe permanent magnet at the back of a telephone 

 disc. The number of pole-pieces and armature-coils on 

 the respective dynamos are such that, on pressing down 

 the keys in succession, the telephone emits the notes of 

 an ordinary musical octave, and by pressing down two 

 or more the compound sound is heard. 



An Englishman finds it somewhat exasperating, if he 

 desires to see the whole Exhibition, to have to be con- 

 stantly taking out his purse to make small payments for 

 entrance here and entrance there ; but, as half the receipts 

 for the shows go to the Exhibition authorities, they will 

 be saved from the financial yfa^yft? that attended the Edin- 

 burgh Exhibition of last year, for that Exhibition had to 

 be finally declared bankrupt, even after all the money 

 guaranteed by the promoters had been called up. Further, 

 the shows are themselves illustrations of the application 

 of electricity to industry and art : the mere bazaar element, 

 that has been so prominent a feature at some of the 

 Exhibitions held at Earl's Court,is practically non-existent 

 at the Frankfort International Electrical Exhibition. 



International, however, the Exhibition is but in name, 

 the comparatively small exhibits of one or two English 

 and American firms only serving as a reminder of the 

 magnificent collections of electrical machinery and appa- 

 ratus England and America could have contributed. As 

 a display, however, of the part Germany is playing in 

 the development of electrical industry, the Frankfort 

 Exhibition is most interesting. 



