VOLTAIC ELECTRICITY. 



\ i I .T A i ( ' K I , KCTRICITY. XL 



ELECTRO- MAO NETS : MODE OF MAKING, AND PROPERTIES 

 HORSE-SHOE MAGNETS. 



We hare already seen the effects produced by an electric oarront 

 ou a niagnotined needle plaoud near to it, and have now to notice 

 the way in which the current is able to produce a magnet. 



Tiio properties of an ordinary magnet uru familiar to mont, 

 and will bo fully explained when treating of Magnetism. We 

 shall find, however, that any piece of iron may be at once con- 

 i temporary magnet by causing a current to pass 

 r..uii.l it. The simplest way of showing this effect is to dip a 

 thin wire, along which a stream of electricity is passing, into a 

 heap of iron filings, when it will be observed that many of them 

 are attracted and cling to the wire as long as the fluid passes, 

 but fall the moment that contact is broken in any part of the 

 circuit. 



The action of the current is, however, seen much more clearly 

 by taking a piece of glass tubing, about three-quarters of an inch 

 in diameter, and 

 twisting a piece of 

 copper wire spi- 

 rally along it, as 

 seen in Fig. 68. 

 Now place a small 

 steel bar or piece 

 of steel wire inside 

 the tube, and allow 

 a current to pass 

 round the spiral. 

 The sleel will at 

 once be converted 

 into a magnet, the 

 power of which 

 will vary according 

 to the strength of 

 the current and the 

 number of coils of 

 wire. If the charge 

 of a Leyden jar be 

 in a similar way 

 passed round the 

 copper wire, the 

 steel will in like 

 manner be magne- 

 tised to a slight 

 extent thus again 

 showing the close 

 alliance between 

 the two kinds of 

 electricity. 



Every magnet 

 has two poles, 

 called respectively 



the north and the Routh, and we shall see that it depends on the 

 direction in which the wire is coiled which end of the bar becomes 

 the north pole. If the wire is made to coil from left to right, so 

 that the current passes round the tube in thtt same direction as 

 the hands of a watch travel, it is said to be a right-handed 

 spiral, the coils of the wire corresponding in position with the 

 threads of an ordinary screw. In this case the end b, at which 

 the current enters, will become a south pole. If, however, the 

 wire bo twisted in the opposite direction, as shown in Fig. 69, 

 it is called a left-handed spiral. In this case the end a, at 

 which the current enters, will be the north, and 6 will be the 

 miuth pole. 



This may be easily remembered by means of Ampere's rule, 

 already given to serve as an aid in remembering in which direc- 

 tion a magnetic needle becomes deflected by the passage of a 

 current near to it. Let the observer imagine himself, or the 

 figure of a man, placed along any portion of the wire, with his 

 face turned towards the centre of the helix, so that the current 

 may enter by his foot and leave by his head ; the north pole will 

 then always be at his loft hand. 



That a magnet may bo made in this way is very easily seen 

 by taking a rod of iron the poker, for instance and twisting 

 a piece of wire round it several times, as shown in Fig. 70. 

 The wire should be covered with cotton or some insulating 



135 K.K 



material, as otherwise a portion of the electricity will pau 

 directly along the iron rod, that being the shorter route. When 

 the current is made to paM along the wire, the poker will b 

 found to have become a magnet, and pieces of iron, such M 

 keys or naiU, held near either end will be attracted and remain 

 suspended. As soon as the circuit in interrupted, and the cnr- 

 rent ceases to flow, the bar, if made of soft iron, will LOM all 

 its magnetism, and the pieces of metal will fall from its ends. 

 If, however, the bar be composed of steel, or even of hard 

 iron, it will be converted into a permanent magnet, bnt may 

 be demagnetised by causicg the current to pass round it in tha 

 reverse direction. 



The electro-magnet derives much of its value from the eas* 

 with which the poles may be reversed, or its magnetism alto- 

 gether laid aside. The iron bar, technically called the eon, 

 should, however, be made of the purest and softest iron that 

 which is known as scrap-iron will be found to answer well 

 Even with this a small amount of polarity is found to remain 

 when the keeper is allowed to come into absolute contact with 



the poles. This U 

 sometimes a dis- 

 advantage, and to 

 guard against it 

 thin discs of paper 

 may be attached 

 to the poles. 



A bundle of thin 

 iron wires, fastened 

 together, is some- 

 times used for the 

 core instead of a 

 solid rod, as it be- 

 comes more power- 

 fully magnetised. 



Either bar mag- 

 nets or horse-shoe 

 magnets may be 

 made by the elec- 

 tric current, the 

 former being most 

 useful when it is 

 desirable to have 

 the poles some 

 little distance 

 apart, while the 

 horse-shoe form is 

 most convenient 

 for showing the 

 lifting power of the 

 magnet, as both 

 poles are then 

 brought to act on 

 the same keeper. 



To make a bar 



electro-magnet, a piece of iron rod about three-quarters of 

 an inch in diameter and twelve or eighteen inches long 

 should bo taken, and copper wire covered with cotton be 

 wound evenly round it, an inch or two of the rod being left 

 uncovered at each end. Much care should be token in winding 

 the wire to guard against the coils overlapping one another, as 

 much of the power would thus be lost. When one layer of 

 wire has been wound on, a second may be laid outside it, wind- 

 ing back again to the end first commenced at, and in this way 

 four or five layers of wire may be used. It will be found ad- 

 vantageous to give a coating of sealing-wax varnish between 

 each layer of wire, as the insulation is thus much improved. 



The power of such a magnet will vary in direct proportion to 

 the strength of the current and the number of coils of wire. 

 If, however, the wire employed be very thin, the additional 

 quantity will cause BO much additional resistance, that a limit 

 to the increase of strength will soon be reached. As a general 

 rule, when the magnet is wanted to attract the keeper from any 

 distance, the wire used in winding should be somewhat thin ; 

 but when it is required to sustain heavy weights, it may be 

 rather thicker. It must not be too stout, however, as it is 

 found that the greatest power is obtained when a certain amount 

 of resistance is offered to the passage of the current, so that it 

 becomes to a certain extent delayed. 



