TPIE POPULAR EDUCATOR. 



Fig. 40. 



to a great number of important practical uses. If we place a 

 small piece of gun-cotton on the wire in Fig. 39, it will be 

 ignited the moment the circuit is completed, and this will 

 be the case whether the connection is made close to the 

 uprights or at a great distance from them. The only thing 

 needed is to let the fine wire come in the circuit, and to let 

 it be complete at every place except the point where the contact 

 is to be made. If the wire be lef b slack, and made to dip into 

 a little heap of gunpowder or coloured fire, they will in like 

 manner be ignited by it. 



Now the practical application of this principle is very simple. 

 Suppose it is required to light a gas-burner from a distance, as, 



for instance, one situated 

 near the roof of a lofty 

 building, we have merely 

 to lead two wires to the 

 burner, and place between 

 their ends a short piece 

 of platinum, so that the 

 jet of gas may play upon 

 it. As soon now as the 

 current is made to pass 

 through this it becomes 

 white-hot, and the gas, on 

 being turned on at the 

 main, is immediately 

 lighted. Fig. 40 will 

 show the way in which 

 the wires may be arranged. 

 A simpler plan, however, 

 is to take a large cork, and 

 cut a hole in the .middle, 

 so that it may slip over 

 the burner. The ends of 

 the battery wires may 

 now be brought, one to 

 each side, and tied with a 

 piece of string, so as to 

 stand a little above the 



cork, and the platinum wire is adjusted between those. A large 

 number of burners may in this way be lighted simultaneously. 



Another application of the same principle is seen in firing 

 cannon from a distance. The platinum wire is made to dip into 

 the powder over the touch-hole, which is thus ignited, and fires 

 the piece. For this purpose fine iron wire may be used in place 

 of platinum ; indeed, it is preferable to it, as the iron melts and 

 burns more easily, and the sparks falling from it render the 

 ignition of the powder more certain. In the same way tor- 

 pedoes may be fired from a distance as soon as one of the 

 enemy's vessels comes over them. Large masses of masonry 

 have also been thrown down in a similar way, a very successful 

 experiment of this kind having been made when removing the 

 massive brickwork of the Great Exhibition of 1862. 



A further remarkable application of the same principle ha.s been 

 made in surgery by the use of the galvanic knife and cautery. 



This is an instrument having somewhat the shape of a knife. 

 The cutting edge, however, is a strip of platinum, and is so ar- 

 ranged that a powerful electric current may be 

 made to pass along it, and thus it may be raised 

 to almost any temperature that may be desired. 

 It is stated that by properly regulating the tem- 

 perature, almost all loss of blood during some 

 operations may be avoided, the heat of the blade 

 completely destroying the tissues and closing the 

 vessels. In many cases this is, undoubtedly, a 

 matter of the greatest importance. In the same 

 way, too, heat may be applied to various internal 

 parts of the body, the current being turned on 

 after the instrument is in its place. In these 

 and all other experiments, when powerful bat- 

 teries are employed, it will be found very convenient to have a 

 wire dipping into a cup of mercury, or some other arrangement 

 for instantaneously shutting off the current when desired. 



The fact already mentioned, that pieces of various metals are 



not only fused but actually volatilised when placed between the 



poles, affords further proof of the heat produced by the current. 



If we twist a piece of fine iron or platinum wire into a spiral 



coil, and immerse it in a wine-glass filled with water, the tempe- 



Fig. 41. 



Fig. 42. 



rature will be raised by the passage of the current, so that 

 in a short time the water may be made to boil. The whole of 

 the spiral must, of course, be immersed, for if any portion of the 

 thin wire be in the air, its heat will not be carried away by the 

 liquid, and it will, therefore, be melted. 



An apparatus for determining the heat produced by the electric 

 current is made in this way . A cork (Fig. 41) is fitted to the 

 mouth of a wide-necked flask, and the tube of a 

 thermometer is passed through a hole in the centre 

 of it, so that the bulb may dip down into the liquid 

 in the flask. Two binding screws are also placed 

 in the cork, and from these pieces of brass wire 

 pass down into the liquid, the coil of fine wire being 

 placed between the ends of them, so as to be 

 entirely in the liquid. The flask is usually filled 

 with alcohol, as this is a worse conductor than 

 water, and also has a less specific heat, so that 

 the same intensity of current would raise it to a 

 higher temperature than it would a similar quantity 

 of water. 



If now we connect the wires from the battery 

 with the instrument, we shall soon see by the 

 thermometer that the temperature of the liquid 

 is increasing. Let us note the time that is required for it to 

 rise any given amount say, for instance, 5. Now double the 

 power of the battery, and let the current pass for a similar 

 lengtn of time. It will be found that the thermometer has 

 risen 20, or four times as much as when the current was 

 only half as strong. We see, then, that the temperature pro- 

 duced in a wire by the passage of an electric current along it 

 increases as the square of the strength of the current. 



Now let us remove the fine wire, and substitute for it one 

 that offers twice the resistance, and, as before, notice the 

 amount through which the thermometer rises in the given time ; 

 it will be found to be just twice as great as before. The re- 

 sistance is doubled, and the heating power is doubled likewise. 

 They increase, therefore, in the same proportion. We see, 

 then, that for exhibiting the thermal effects of the current the 

 wire should be made of a badly-conducting material, and should 

 be as small as practicable. 



Before leaving this class of effects, it will be well just to 

 notice the construction of the electric fuzes usually employed for 

 mining purposes, or for firing ordnance. A good model of one 

 may be made by taking a small phial (Fig. 42) capable of 

 holding about half an ounce. Make a small hole through the 

 cork, and pass through this the ends of two insulated wires, 

 which should be twisted together for a short distance. The 

 inner ends should now be carefully cleaned, and about an inch 

 of fine iron wire placed between them, the ends 

 being carefully fastened, or, better still, soldered 

 to the thicker wires. The bottle may now be 

 filled with gunpowder, and we have a fuze ready 

 for use. If it is to be placed under water, the 

 cork should be covered with sealing-wax, or some 

 other waterproofing material, so as to keep the 

 powder dry. 



The fuzes usually employed are made of short 

 pieces of brass tube closed at each end by discs 

 of wood or leather, but are otherwise just like 

 that shown here. There is, however, another 

 variety in which the explosive powder itself is 

 the imperfect conductor. These are charged 

 with a mixture of chlorate of potash and sub- 

 sulphide and subphosphide of copper ; the latter 

 of these is a conductor, and the proportions of the different in- 

 gredients are so arranged that this shall become heated by the 

 current and ignite the compound. In this kind of fuzo the fine 

 wire is dispensed with, and the ends of the battery wires are 

 brought so close together as almost to touch, so thab the current 

 has to pass through but a very small portion of the mixture. 

 The usual way in which this is accomplished is by bending the 

 piece of wire in the middle, and putting the two ends of it 

 through one cap of the fuze. When the wire is thus fastened, 

 a narrow slit is made through the bend by means of a fine saw 

 in the way shown in Fig. 43 ; the fuze is then filled with the 

 composition, and the flame from it will ignite gunpowder with 

 certainty. 



There is another kind of fuze known as Statham's. A piece 



Fig. 43. 



