Xl8 Progress in Physics. [January, 



metallic stud, M ; the other side of the receiving instrument is connected with 

 the earth at n. When the key is depressed, contact is broken at m, and the 

 condenser, d, is placed in communication, through the stud, h, with the wave- 

 generating apparatus beyond r. A convenient condenser to use is one of three 

 microfarads. 



The vibrations of the tuning-fork, a, Fig. 21, are employed to set up the 

 waves, and the fork is caused to vibrate incessantly by power obtained from the 

 battery, b, one pole of which battery is connected with the fork ; the two light 

 springs, e f, have small platinum studs at the end, and when the fork is at rest, 

 the light piece of metal, r, attached to one of its legs, is in contact with the 

 spring, e. A current then passes from e, round an electro-magnet, the poles of 

 which are shown at h k. Instantly, of course, the prongs of the fork are pulled 

 outwards by the attraction of the magnet, contact with e is broken, and with f 

 established ; the fork, however, at once springs back again, and thus continuous 

 vibrations, at the rate of about 200 per second, are produced. The battery 

 power used for generating the waves is ten-quart Daniell's cells. Some 

 electro-magnetic apparatus is fixed between the tuning-fork and the condenser, 

 and this apparatus sends the impulses into the condenser when the key for 

 sending signals is depressed. 



The principle of construction of the receiving instrument is shown in Fig. 22. 

 A d b e is the iron frame of a sounding-board, which board is 4 feet 6 inches 

 long between the bridges, h h. The thick iron wire, a b, is No. 14 gauge, and 

 its tension is regulated by the two screw nuts shown at the ends of the iron 

 frame. This wire passes through the hollow helix, n, which helix is 3 inches 



Fig. 22. 



H 



long and } inch internal diameter. This coil is attached to the iron frame, and 

 does not touch the sounding-board. Two horse-shoe magnets, k k, are placed 

 one on each side of the wire. One end of the coil, n, is connected with the con- 

 denser while a message is being received, and the other is connected with the 

 earth. The rapid currents from the condenser magnetise the wire within the 

 helix, so that it is attracted or repelled by one of the two magnets, k k. By 

 this means rapid vibrations are set up in the wire after the wire has been 

 " tuned," by turning the screw A or b, so that it has a tendency to vibrate 

 synchronously with the pulsations of the fork at the sending station. When 

 it is properly tuned, very feeble currents will give a distinctly audible sound. 



There are various ways of augmenting the sound if desired, by means of 

 vibrating tongues, strained diaphragms, the use of the stethescope, and other 

 appliances ; the signals can also be made visible by optical apparatus, or be 

 made sensible to the touch. In the experiments above mentioned the sound 

 was full and clear, without such additional aids. There are also various ways 

 of originating waves in the sending portions of the instrument. 



By this apparatus, while messages are being transmitted from London to 

 Brighton by the ordinary Morse apparatus, through one line wire, the singing 

 instruments may be placed at half a dozen intermediate stations on the same 

 wire, and each station may communicate with its next neighbour, without 

 interfering with the main line work. The singing instruments on one length of 

 the wire are prevented from interfering with those on the next length by the 

 interposition of an electro-magnet, for an electro-magnet offers at the first 

 moment considerable opposition to the passage of a current of electricity, so 

 may be considered to be opaque to rapid waves. By the arrangement just 

 described, half a dozen or more messages may be traversing the same wire at 

 the same time. Or, if there be no intermediate stations, two messages at the 



