Aug. 1st, 1887.] 



SCIENTIFIC NKWS. 



^33 



Every one who has played with a penny magnet will see 

 that the magnet A must be attracting the centre of the dia- 

 phragm C, and also that any variation in the strength of 

 that magnet will either bring C nearer to, or allow it to 

 spring farther from A. Now the wire in the coil B is all 

 at right angles to A, and by our second law, any current 

 passing in it will tend to magnetise A in one direction or 

 the other, according to the current's direction. If the 

 tendency is to magnetise A in the same way as its per- 

 manent magnetism, it will be strengthened ; if the tendency 

 is to reverse its magnetism, it will be weakened. Then, if 

 we can produce a current in the coil which shall change its 

 direction and strength in a way corresponding to the shape 

 and frequency of certain sound waves, we shall cause the 

 diaphragm C to perform vibrations correspondingly, and it 

 will give forth to the surrounding air the sound desired. 

 But how is this vibratory current to be produced in the 

 coil ? Here our first law comes in. The coil B is in a 

 magnetic field of a certain strength, i.e., a certain number of 

 magnetic lines of force pass through it. If the diaphragm C 

 approaches the magnet A, the number of lines of force pass- 

 ing through B is increased ; if C recedes from A, the 

 number of lines passing through Bis diminished. In either 

 case an electrical current is set up in B, and in the whole 

 circuit (including the coil of the distant instrument), of which 

 it forms part, but the currents will be in reverse directions 

 in the two cases. If this chain of statements is borne in 

 mind, it will be immediately obvious that any vibrations im- 



Fig. 3. — Section of Graham Bell's Telephone. 



parted to the transmitting diaphragm from the air, will 

 set up vibratory electrical currents in the electrical circuit 

 which includes the coils of both instruments, and that the 

 current vibrations will correspond to the sound vibrations 

 in force and direction. Here, then, is the vibratory current 

 which we have just shown to be capable of causing the re- 

 ceiving diaphragm to produce sound. Put the two actions 

 together, and the transmission of speech by two Bell's tele- 

 phones, one acting as a transmitter at one end of the line, 

 and the other as a receiver at the other, is explained. 



It will be seen that no battery or other external source of 

 current is used with the Bell instruments. The whole of 

 the energy is supplied by the original air vibrations ; the 

 faint but clear whisper which issues from the receiver is 

 in very truth produced by the exertion of the person speak- 

 ing to the transmitter. Few prettier examples of the trans- 

 formation of energy can be given than the one just de- 

 scribed, which may be thus tabulated — 



i transformed to diaphragm vihrations- 



Eleclrical Energy : 



ed to -vibratory change in a magnetic field- 

 Transformed to vibratory current- 

 Transformed to vibratory change in magnetic field, causing vibratory attracti( 

 Mechatikal Energy : 



A complete cycle of changes of form, yet, all through, the 

 one characteristic — the vibratory feature — is maintained. 

 {To be conlimted.) 



BATHYMETER, OR SOUNDING 

 APPARATUS. 



IN our March number we described an apparatus used for 

 deep-sea sounding, and we now illustrate a much 

 smaller and less complicated apparatus, used chiefly for 

 taking flying soundings, without stopping the ship or 

 altering her speed. This instrument is called by its makers 

 the Bathymeter (from ftnOo^ bottom and ;(ct/>oi' measure), and 

 its action depends on the pressure of the water overlying 



% 



% 



I r. 



3 



Apparatus for taking Flying Soundings at Sea. 



it when it is hove overboard, as will be presently described. 

 Fig. I represents an external view, and Fig. 2, a vertical 

 section. The sounding-line is a thin wire cord of great 

 tensile strength, made of phosphor-bronze to prevent rust 

 or corrosion. This line is fastened to the eye a, and at 

 the other end a sinker is attached to the eye b. Small 

 holes c c are provided in the outer casing, and through 

 these the water passes to the interior, and there presses 

 on a hollow corrugated tube d, hermetically sealed. On the 

 top of this tube there is a catch c, acting on a lever /, and 

 this in turn works the needle or pointer g, on the dial 

 face. 



