4i6 



NATURE 



[March 2, 1893 



while they are of interest, are not, up to the present, 

 siutable for exhibition. 



Let me now return to single spark photographs. We 

 have seen that the magnesium flash, which for the 

 purpose of portraiture is practically instantaneous, yet 

 fails to appear so when so moderate a speed as forty 

 miles an hour (and indeed a far lower speed) is used 

 for the purpose of examining it. Is anything of the 

 kind true in the case of the electric spark? Will 

 the spark, by which we saw the clock-face abso- 

 lutely sharp, after all fail to give a sharp view 

 when tested by a much higher speed ? I have taken such 

 a spark and attempted (though I knew what the result 

 would be) to photograph by its light the bullet of a 

 magazine rifle passing by at the rate of about 2100 feet a 

 second, or, what is the same thing, at about 1400 miles an 



hour ; the result (Fig. i) shows not a clear sharp bullet but 

 a blur ; the spark lasted so long a time that this bullet was 

 actually able to travel half an inch or so while the 

 illumination lasted. Thus we see, that if we wish to 

 examine bullets, &c., in their flight, any electric spark will 

 not necessarily do. We shall have to get a spark which 

 while it gives enough light to act on the plate yet lasts so 

 short a time that even a rifle bullet cannot move an 

 appreciable distance during the time that it is in existence. 

 A knowledge of electrical principles enables one to 

 modify the electrical apparatus employed to make this 

 spark in such a manner that its duration may be greatly 

 reduced without, at the same time, a very great sacrifice 

 of light ; but while this may be done it is important to 

 be able to observe how long the spark actually lasts, when 

 made by apparatus altered little by little in the proper 

 manner. The desired information is at once given by the 

 revolving mirror. For instance, every one is aware how, 

 by a turn of the wrist, one may reflect a beam of sunlight 

 from a piece of looking-glass so as to travel up the street 

 at a most tremendous velocity ; but suppose that, instead 

 of being moved by a mere turn of the wrist, the mirror is 

 made to rotate on an axle by mechanical means at an 

 enormous speed ; then, just as the rotation is more rapid, 

 so will the beam of light travel at a higher speed. In the 

 particular case that I am going now to bring before 

 your notice, a small mirror of hardened steel was made 

 by Mr. Colebrook, the mechanical assistant in the physical 

 laboratory at South Kensington, mounted so beautifully 

 that it would run at the enormous speed of 1000 turns a 

 second (not 1000 a minute) without giving any trouble. 

 The light from the spark wasfocussed by the mirror upon 

 a photographic plate. Now if the light were really instant- 

 aneous, the image would be as clear and sharp as if 

 the mirror were at rest ; if, on the other hand, it lasted 

 ong enough for the image to be carried an appreciable 

 distance, then the photograph would show a band of light 

 drawn out to this distance. The mirror is now placed on 

 the front of the platform, and a beam of electric light is 

 focussedbyit upon the screen, from which it is distant 

 about 20 feet. Now that I turn the mirror slowly, you 

 see the spot of light drawn out into a band reaching across 



NO. 1 2 18, VOL. 47] 



the screen, and this is described over and over again as 

 the mirror revolves. Let us suppose that the mirror is re- 

 volving once a second, then it is easy to show that the 

 spot of light is travelling at about 250 feet a second. It is 

 not difficult therefore to see that if the mirror is revolving 

 1000 times as fast, the spot of light will traverse the screen 

 1000 times as fast also, i.e., about 250,000 feet a second, 

 or 160,000 miles an hour — a speed which is 200 times as 

 great as that of a Martini-Henry bullet, while such a bullet 

 only travels 14 times as fast as an express train. You will 

 see, then, that it is not difficult to observe how long a spark 

 lasts when its image can be whirled along at such a speed 

 as this, I have now started the electro-motor, and the 

 mirror is turning more and more rapidly. Now it gives 

 a niusical note of the same pitch as that given by the 

 tuning-fork I am bowing ; it is therefore turning 5 1 2 

 times a second. It is now giving a higher note, i.e. it is 

 turning faster and faster, until at last it gives the octave, 

 at which time it is turning 1028 turns a second. The 

 band of light on the screen is produced by a spot now 

 travelling at a still higher speed than that which I have 

 just mentioned. I had hoped to have shown with this 

 apparatus the actual experiment of drawing out the ap- 

 parently instantaneous flash of an electric spark into a 

 band of light, but I found that while it was easy to show 

 the experiment in a small room, the amount of light was 

 not sufficient to be seen in a great room like this. I must 

 therefore be content to show one or two of the photo- 

 graphs which were taken lately in the physical laboratory 

 at South Kensington by two of the students, Mr. Edser 

 and Mr. Stansfield, whom I now take the opportunity of 

 thanking. The next slide shows the drawn-out band of a 

 particular spark made between magnesium terminals by 

 the discharge of a condenser of i\ square feet of window- 

 glass, the spark being \ inch long. Below the drawn-out 

 band I have drawn a scale of millionths of a second. 

 If the spark had been instantaneous it would have ap- 

 peared as a fine vertical line. This line, however, has 

 been drawn sideways to an extent depending on the dura- 

 tion of the spark. The spark, except at theends, is extinct 

 in rather less than one-millionth of a second, but the 

 ends remain alight like two stars, being drawn out in 

 consequence into two lines, which have lasted, as 

 measured by the scale, as long as six or seven millionths 

 of a second. Such a light is, therefore, seen to last 

 when tested with this very powerful instrument so long 

 that it seems absurd to call it instantaneous. It lasts too 

 long for the purpose of bullet photography. In order to 

 get sparks of shorter duration it is necessary to abolish 

 the metal magnesium, in spite of the brilliant photo- 

 graphic effect of the two ends of the spark between 

 knobs of this material, it is well to avoid all easily volatile 

 metals, such as brass, because of the zinc that it contains, 

 and instead to employ beads of copper or of platinum. In 

 the second place, the duration of the spark proper, which 

 in the last case was nearly a millionth of a second, can 

 be reduced by (i) reducing the size of the condenser, but 

 one must not go too far, as the light is reduced also ; 

 (2) by replacing any wire through which the discharge 

 may have taken place 

 by broad bands of 

 copper as short as 

 possible, this has the 

 further advantage of 

 increasing the light ; 

 and (3) the light may 

 be increased without 

 much change being 

 made in the duration 

 by making a second 

 gap in the discharge 

 circuit, the spark in 

 which, however, must be hidden from the plate. Fig. 2 

 shows the trail of the best spark for the purpose of bullet 



