328 



THE POPULAR EDUCATOR. 



^deration that the telephone gives no sound so long as a steady 

 current traverses the circuit of which it forms part, but that it 

 is necessary for the current to experience rapid changes or 

 variations. Then sounds are bransmitted by it. He argued 

 that if an electric current could be thus varied by varying the 

 intensity of a beam of light inpinging upon a selenium cell and 

 that if such variations could be made to correspond to the 

 variations of the air produced during the utterance of vocal or 

 other sounds, the telephone could be made to reproduce such 

 sounds. 



We may pause here for a while to describe a wonderful little 

 piece of apparatus contrived some years ago by Dr. Siemens, 

 for it illustrates in a very perfect manner the sensitiveness of 

 selenium to light rays. This is known as Dr. Siemens' selenium 

 eye. It is in reality a miniature human eye, with a lens in 

 front, and lids to close when it is weary, for, strange to say, it 

 does, like its perfect prototype, become weary when long ex- 

 posed to bright light. The lens causes any light to which the 

 eye is exposed to be concentrated in the interior of the eyeball, 

 and at this spot is placed a selenium grating. This grating, no 

 larger than a threepenny-piece, is made of two fine wires run- 

 ning together in zigzag fashion, but not actually touching one 

 another. Upon these wires is placed a melted drop of selenium, 

 and the ends of the wires are joined up with a galvanometer 

 and battery. If the eye has been closed and at rest for some 

 time, it is sensitive to the smallest gleam of light, even that 

 little which can be reflected into it from a blackened sheet of 

 paper. But if it has been exposed to a bright light, the lids 

 must be closed for a long time before it is again sensitive to 

 feeble rays. Such experiments as these show the great sensi- 

 tiveness of selenium, but Professor Graham Bell has prepared 

 cells giving far more wonderful results. 



Before the selenium is in a fit state for these delicate experi- 

 ments, it requires to go through a process of annealing. The 

 old plan of doing this was to place the cell (the structure of 

 which it is not necessary here to describe) in a vessel of linseed 

 oil, together with a thermometer, at the same time connecting 

 it with a battery and galvanometer, the whole being heated 

 over a gas stove. After a heat is reached of about 210 C., the 

 cell is kept at that temperature for several hours, being after- 

 wards packed up in a box so that it would cool down very 

 gradually to the temperature of the air. The entire operation 

 occupied about three days. In the modern form of cell adopted 

 as the best by Professor Bell after trying a number of different 

 patterns', this, long annealing process is done away with, and the 

 same effect ia produced in a few minutes. 



This new form of cell is cylindrical in shape, and is in appear 

 ance not unlike a reel of c ,tton. It is made of a number of 

 discs of brass, separated by slightly smaller discs of mica, so 

 that when the cylinder is joined up, these differences in dia- 

 meter constitute a number of grooves round its surface. 

 These grooves, about one hundred in number, are filled in with 

 selenium. By means of two bolts passing through the cylinder 

 from end to end the discs are placed in metallic connection, the 

 discs 1, 3, 5, 7, &'?., being in communication with one bolt, the 

 even numbers being placed in like relation to the other bolt. 

 These may be regarded as the terminals of the cell, allowing it 

 to be connected electrically with the telephone and other neces- 

 sary apparatus. 



The annealing process is very simply managed. The little 

 cylinder is kept in rotation in a lathe while a gas flame is burn- 

 ing beneath it, but separated from it by a metal plate.. The 

 brass gradually gets hot enough to melt a stick of selenium 

 when applied to it, and is thus covered all over with that sub- 

 stance and allowed to cool. So far it is a non-conductor, or at 

 least may be said to have a very high resistance. To make it 

 tractable, it is once more re-heated over a gas stove, and now 

 a strange change comes over it. The black amorphous sub- 

 stance becomes crystalline, and looks like a metal. The heating 

 is continued until the substance shows signs of melting, when 

 the operation is concluded, and the selenium is sensitive to 

 light. It may be mentioned here that the cylindrical form of 

 cell is the most convenient for introducing into the focus of 

 a parabolic reflector, in which situation it is placed in all these 

 experiments. 



In our next lesson we propose to explain how this selenium 

 cell was utilised by Professor Graham Bell in the construction 

 of the photophone. 



ACOUSTICS. IV. 



LAWS OF VIBRATING STRINGS SONOMETER MARLOYE'S HAR? 

 VIBRATING PLATES SAND FIGURES. 



WE will now direct our attention to the vibrations of strings 

 or cords, and inquire into the laws which govern them. This 

 subject is an important one, as many of our musical instrument? 

 consist merely of strings, which are made to vibrate. The 

 apparatus usually employed in these investigations is called a 

 monochord or sonometer, and is represented in Fig. 15. It con- 

 sists essentially of a single wire or cord, m n, the length and 

 tension of which can be altered at pleasure. One end of this is 

 fixed to a peg at the extreme left of the instrument ; the other 

 end passes over a pulley, and has a number of weights suspended 

 from it, by means of which the tension can be altered at plea- 

 sure. Two bridges, o and n, are placed under the cord, ont 

 near each end; these form its virtual extremities, and rest upon 

 the hollow sounding-box which forms the base of the instrument. 

 When the wire is set in vibration, the pulsations are conveyed 

 through these bridges to the sounding-box, and thus to the body 

 of air contained in it. In this way the power of the sound is 

 very materially increased. 



If we were merely to suspend the cord from a fixed hook, 

 placing a weight at the lower end to keep it stretched, and then 

 to set it in vibration, we should easily discern its vibration by 

 the eye, but scarcely any sound would be produced, as there 

 is no vibrating body to which its motion would be imparted. In 

 the sonometer the cord vibrates in just the same way, but the 

 sounding-box enables us to hear as well as to see the vibrations. 



At the back of the instrument is a rod, on which the distance 

 between TO and n is divided into one hundred equal parts, and a 

 movable bridge, o, can be placed at any part of this, so as to 

 touch the string in any required place, and damp its vibrations 

 there. 



If now we remove o altogether, and pluck the string in the 

 centre, or draw a violin-bow across it, we shall obtain a sound 

 which is the fundamental note of the string, the whole of which 

 will be thrown into vibration, as shown at A (Fig. 16). Now 

 place the bridge, o, at the division of the scale marked 50 

 that is, midway between m and n and excite one division of 

 the string by means of the bow, as before. Both parts will at 

 once be thrown into vibration, and the cord will present the 

 appearance shown at B ; but the note produced will be found to 

 be just an octave higher than the fundamental note of the string. 



Now move the bridge, o, to nearly the division marked 33, so 

 as to bo one-third of the way along the cord, and draw the bow 

 across the segment, a b. We shall now obtain the note a fifth 

 higher than the octave, and the portion b d of the cord (Fig. 

 1 6, c) will be seen to be divided into two ventral segments, as 

 they are termed, separated by a node, c. The existence of this 

 may easily be shown by placing three bent pieces of paper 

 astride the cord at the points e, c, and /, and then exciting it 

 as before. Those placed at e and / will at once be jerked off, 

 while that at c, being placed at a node, will remain unmoved, 

 showing that the cord there is at rest. 



By moving the bridge to the division 25, we shall find the 

 whole length of the cord divided into four segments (Fig. 16, D). 

 The sound produced in this case will be just two octaves above 

 the fundamental note. The division of the cord may be rendered 

 manifest, as before, by placing pieces of paper on the wire. 



In these experiments we may dispense with the bridge alto- 

 gether, and damp the cord at any required place by lightly 

 touching it with a feather. As a result of them all, we find that 

 the shorter the vibrating segments are, the higher will be the 

 note produced. By diminishing their length a half we raise the 

 ncte an octave, and, as we have already seen, this is produced 

 by doubling the number of vibrations in any given time. We 

 thus obtain the following fundamental rule : The number o* 

 vibrations in the same time varies inversely as the length of ilia 

 siring, the tension remaining unaltered,. 



The next thing that modifies the note produced by a strirf 

 is its tension. Experimental proof of this fact can easily be 

 obtained by altering the weight at w, or, easier still, by varying 

 the pressure by the hand. By carefully experimenting in this 

 way we shall find that, by increasing the tension fourfold, we 

 raise the note an octave, that is to say, we produce double the 

 number of vibrations. The second general law, then, may be 

 stated as follows ; The nwtriboff of vibrations made by the cord 



