282 



ELECTRICITY. 



off the magnet and placed on the end of a cylinder of 

 pasteboard, which serves as a support in bringing it 

 quite close to the under surface of a cast-iron plate 

 (1 foot diameter, i-inch thick), which has been heated 

 over a large Bunsen-flame. Thus the shellac is 

 uniformly heated, and the iron tilings, absorbing 

 the radiation, sink into the softened film and are 

 "fixed." 



I generally allow the heat to act until the metallic 

 lustre of the filings has disappeared, by sinking into 

 the shellac, and the film appears quite transparent. 

 This degree of action is necessary when photographic 

 prints are to be made from the plate, but when they 

 are to be used as lantern-slides I do not carry^ the 

 heating so far. After the plate has cooled, it is 

 allowed to fall upon its ends, on a table, so that any 

 filings which have not adhered may be removed. 



By this process many plates have been produced, 

 showing the action of single magnets of various 

 forms and of juxtaposed bars, as well as the effects 

 of electric currents led by wires through holes drilled 

 in the plates. Those exhibiting the inductive action 

 of magnets on bars of soft iron and the interaction 

 of magnets and electric currents are peculiarly inter- 

 esting. An approximate representation of the result- 

 ant lines of the terrestrial magnetic action has been 

 obtained by magnetizing equably-tempered steel 

 disks of from 2 inches to 3 inches, and even more, in 

 diameter. The magnetic axis or axes of these disks 

 are predetermined by making them the continuations 

 of the axes of very powerful electro- magnets, termi- 

 nated with cones of soft iron with slightly-rounded 

 apices. The arcs of the great circles, including the 

 terrestrial magnetic poles, having been calculated, the 

 axes of the electro-magnets are inclined to that angle, 

 while the steel disk is held close to their poles. On 

 passing the current the disk is magnetized, and we 

 have an approximate representation of a section of 

 the earth's magnetic effect. 



Electric Clocks. Improvements in the elec- 

 tric clock are claimed to have been made by 

 Messrs. Bonhomme and Milde", of Paris, by 

 which time is indicated with great accuracy, 

 and the hours are struck under all conditions 

 of the weather, while any number of clocks 

 and striking motions may be made to work 

 simultaneously, and in perfect unison, when 

 connected with the regulating or primary 

 clock. The primary clock, worked solely by 

 electricity, is situated in any desirable place, 

 and put in electric communication with the 

 other clocks, and transmits its precise motion 

 to them. The clocks are each provided with 

 electro-magnets, which act on armatures, and 

 cause the latter to vibrate through a limited 

 space (controlled by an adjusting screw), when- 

 ever the electric pulsations take place. This 

 is effected by making and breaking the elec- 

 tric circuit. To these vibrating armatures, or 

 to arms and levers connected therewith, are 

 attacked ratchet-clicks, which act on a central 

 ratchet-wheel, and every time they are moved 

 forward they pull round the ratchet to a cer- 

 tain extent, and thus act upon a central toothed 

 wheel, which is mounted on the same shaft 

 and gears into and actuates the clock-work 

 which carries round the hands. The escape- 

 ments are acted on in liko manner, and the 

 time is controlled by a pendulum, which is in- 

 sulated by means of ivory blocks. 



Improved Telegraph- Cables. In manufac- 

 turing the Dublin and Holyhead cable, it was 



desired to make it of unusual strength and 

 working capacity. There were seven con- 

 ducting wires, each one being made of a strand 

 of seven copper wires, one of which formed 

 the centre round which the other six were 

 stranded. Each one of the stranded wires, 

 when completed, averaged 107 Ibs. to tho 

 nautical mile, and was so well insulated that, 

 when the conductivity of the wire was com- 

 pared to that of the insulating or non-conduct- 

 ing material, it stood as 4,350,000 to 1, indi- 

 cating a core of a highly superior character, 

 and a most perfect preparation of the insula- 

 tory material. The latter was a series of coat- 

 ings of "hot compounds," and plastic gutta- 

 percha, each put on three times alternately. 

 The diameter of the completed main cable is 

 2J inches, and its weight per knot 13.75 tons. 

 It was safely submerged, and, upon testing, 

 showed highly-satisfactory results, the mean 

 connection of the seven wires being TOO me- 

 gohms per knot, reduced to the standard tem- 

 peratures of 75 F., the mean temperature of 

 the sea-bottom being proved by the electrical 

 tests to be 46 F. This was the first cable 

 laid by the English Post-office Department 

 after its acquisition of the telegraphic lines of 

 the kingdom. 



Electric Torpedoes. By an invention due 

 to Captain A. Noble, of England, any number 

 of submarine torpedoes may be exploded with 

 only one wire. Formerly, it had been neces- 

 sary to connect each torpedo with the firing 

 battery by means of a distinct insulated wire 

 or cable. On Captain Noble's plan, the main 

 insulated cable is connected with a number of 

 branch insulated wires corresponding to the 

 number of torpedoes to be laid down. These 

 torpedoes are constructed so that any one of 

 them is exploded by a passing ship by the 

 completion of an electric circuit through tho 

 insulated wire. Upon the explosion of any 

 one of the torpedoes, it is at the same moment 

 disconnected from the electrical arrangement, 

 so that its explosion in no wise interferes with 

 the liability of any other of the torpedoes to 

 be subsequently exploded by collision with a 

 vessel. In practice, Captain Noble makes tho 

 electrical communication between each tor- 

 pedo and the insulated wire through an Abel 

 fuze, which, together with the joints that con- 

 nect it with the torpedo and the insulated 

 cable, is enclosed in a water-tight casing. 

 When the torpedo is exploded by completing 

 the electric circuit, the fuze enclosed in the 

 insulating box is simultaneously exploded 

 thereby, and the electric communication of 

 that particular torpedo with the insulated wire 

 is instantaneously severed. The saine result 

 he also obtains with a platinum or fine iron 

 wire, or any other electrical fuze, in place of 

 the before-mentioned kind. 



Electric Signals in Mines. In Silesia, "West- 

 phalia, and the basin of the Saar, the use of 

 electric signals for mines is becoming very 

 general. The Mechanics' Magazine describes 



