ELECTRICITY. 



283 



the operation of these signals in the Graf- 

 Beust mines at Essen. Their characteristic is, 

 that the voltaic circuit is closed by the earth. 

 Alon gthe circuit are interposed alarums, visi- 

 ble at every stage of the mine; the current 

 is constantly closed; the alarums sound when 

 the working of the manipulator interrupts the 

 current; the pile is composed of twenty ele- 

 ements of brass-zinc, plunged in a solution of 

 sulphate of magnesia. At the Rhein-Elbe mine 

 the system is different. In the transmission 

 of signals from the bottom to the surface, a 

 complete circuit is made between the two 

 points. The manipulator is composed of a 

 wooden fork, the teeth of which are covered 

 in the interior with two sheets of copper in 

 contact with the conducting wires; at the 

 lower extremities of the sheets there are two 

 metallic pendants which are brought into con- 

 tact at the moment of interrupting the cur- 

 rent, by closing the teeth of the fork, when 

 the passage of the current sounds the bells 

 placed at the surface. To transmit signals 

 from the surface to the bottom, it is necessary, 

 of course, to have a manipulator at the sur- 

 face and bells at the bottom, but a single con- 

 ductor only is required to connect them. The 

 circuit is closed by the wire attached to the 

 bells. The pile used at Ehein-Elbe is com- 

 posed of six elements of zinc-charcoal plunged 

 in a stratum of mercuric sulphate, renewed 

 every two months. The wires are protected 

 by a wooden sheath. The erection in a wooden 

 shaft of 220 yards deep costs almost 40, in 

 which sum are comprised the expense of ma- 

 terials required for a year, and two spare 

 elements. In a dry shaft the conducting wire 

 can be covered simply with gutta-percha, in 

 which case the cost is reduced. 



Electric, Time- Guns. The Mechanics' Mag- 

 azine gives an interesting account of the prac- 

 tical working of the electric time-guns, now 

 regularly fired at a given hour at Edinburgh, 

 Newcastle, and North Shields; true mean 

 time is given for those points from Greenwich. 

 At Edinburgh an electric current is passed 

 direct from the Royal Observatory clock to 

 the castle-clock, which works directly in per- 

 fect unison with the former. Attached to the 

 movement of the castle-clock is a detent and 

 lever arrangement, which is liberated by the 

 mechanism of the clock at the precise moment 

 necessary for the discharge of the gun (1 P. M.). 

 This is accomplished by the fall of a lever, 

 pulling a cord attached to a friction-tube 

 placed in the touch-hole of the gun. The daily 

 gun alike controls the accuracy of railway, 

 church, and watch time-pieces, and is relied 

 on for miles around when the flash can be 

 seen. The sound travels over a radius of ten 

 miles, and, by allowing as a correction for the 

 sound one second for each quarter of a mile, 

 the signal becomes equally available for all 

 points within its hearing. At Newcastle the 

 gun is fired by a direct current from the Edin- 

 burgh Observatory clock, 120 miles off, the 



charge being ignited by the passage of tha 

 electric spark direct into the fuze. Practically, 

 the interval of time taken from the passing of 

 the current to the ignition of the powder and 

 the discharge of the gun is found .o be about 

 one-tenth of a second only, a time so small as 

 to be virtually of no importance. 



Nature of the Luminous Arch. In passing 

 a current of electricity through highly-rarefied 

 atmosphere in a sealed tube, placed between 

 the poles of an electro-magnet, a luminous 

 arch is produced which follows the course of 

 the magnetic rays. As the electro-magnet is 

 magnetized, the tube, which before was full 

 of a luminous cloud, is seen gradually to ex- 

 hibit a change: the magnet gathers up this 

 diffused cloud, and builds up the arch. Mr. C. 

 F. Varley, in a paper read before the Boyal 

 Society, details as follows the results of his 

 experiments to explain these phenomena. He 

 says: "To endeavor to ascertain the nature 

 of this arch, a special tube was constructed. 

 A piece of talc, bent into the form U, had a 

 fibre of silk stretched across it ; on this fibre 

 of silk was cemented a thin strip of talc, 1 

 inch in length, T Vth inch broad, weighing 

 about ^th of a grain. The tube was sealed 

 up and exhausted; carbonic acid and potash 

 were used to get a high vacuum. When the 

 magnet was not magnetized, the passage of 

 the current from wire to wire did not affect 

 the piece of talc. When the magnet was 

 charged, and the luminous arch was made to 

 play upon the lower portion of the talc, it re- 

 pelled it, no matter which way the electric 

 current was passing. When the tube was 

 shifted over the poles of the magnet so as to 

 project the luminous arch against the upper 

 part of the talc, the upper end of the talc was 

 repelled in all instances ; the arch, when pro- 

 jected against the lower part of the talc, being 

 near the magnet, was more concentrated, and 

 the angle of deviation of the talc was as much 

 as 20. When the upper part of the arch, 

 which was much more diffused, was thrown 

 upon the upper part of the talc, it was repelled 

 about 5. This experiment, I think, indicates 

 that this arch is composed of attenuated par- 

 ticles of matter projected from the negative 

 pole by electricity in all directions, but that 

 the magnet controls their course, and these 

 particles seem to be thrown by momentum on 

 each side of the negative pole, beyond the 

 limit of the electric current. This arch re- 

 quires time for its formation, for, when a 

 charged condenser is discharged through tho 

 tube, no arch is produced. The arch from tho 

 negative pole is a hollow cylinder ; the little 

 talc tell-tale against which the arch was pro- 

 jected cut out the light, and a corresponding 

 dark space existed throughout the remainder 

 of the course of the arch. There was on the 

 talc, at the spot where the arch struck it, a 

 little bright luminous cloud, as though the 

 attenuated luminous vapor was condensed by 

 this material obstruction. Great care had 



