CHAMBERS'S INFORMATION FOR THE PEOPLE. 



graduated circular rim. The whole is protected 

 by a glass cover from the disturbance of the air, 



Fig. 26. 



and rests on a stand supported by three screws, 

 for levelling the instrument, that the needles may 

 hang free. When used, the bobbin is turned 

 round by the screw, Q, until the needle stands at 

 the zero-point ; and the wires through which the 

 current is sent are fixed to the binding-screws, s, /, 

 which communicate with the ends of the bobbin 

 wire. The number of degrees that the needle 

 deflects may then be read off, and this gives an 

 idea of the strength of the current. It is to be 

 observed, however, that twice as great a deflection 

 does not shew twice as strong a current, because 

 for different angles of deflection the coil is differ- 

 ently situated towards the needle. Still, we are 

 sure that a greater deflection indicates a stronger 

 current; and for about 20 on each side of the 

 zero, the strength may be taken as proportional 

 to the angle of deviation. 



The third form of galvanometer multiplier is 

 called the Differential Galvanometer. It is used 

 for comparing the strength of two currents, and 

 is a simple modification of the last. 



For the description of other forms of current 

 measurers, such as the Tangent Galvanometer and 

 the Sine Galvanometer, the reader may refer to 

 any of the larger treatises on the subject. 



The Reflecting Galvanometer of Sir William 

 Thomson is by far the most delicate of all our 

 galvanometers. It was invented by him in con- 

 nection with the Atlantic telegraph, and we shall 

 describe it under Submarine Telegraphy. 



Measure of Current Resistance. Bodies differ 

 to an almost incredible degree in the resistances 

 they offer to the passage of a current. When a 

 current has to face great resistance in its course, 

 part of its power is spent in overcoming this 

 resistance, and disappears as heat. Thus the 

 current is weakened in the same degree as it is 

 resisted ; and it is interrupted when it is wholly 

 unable to overcome the resistance in its path. 



The determination of the relative resisting 

 powers of metals is thus a matter of great import- 

 ance. But it is also a matter of extreme difficulty, 

 for temperature .and purity of metal have a great 

 effect on it. Thus th per cent, of iron present in 



274 



copper will increase its resistance to the current 

 by as much as 25 per cent. Silver offers the least 

 resistance of all our conductors, and copper the 

 next smallest. The other metals follow in this 

 order gold, zinc, platinum, iron, tin, lead, mercury. 

 Between silver and mercury there is a wide differ- 

 ence in resisting power ; mercury offers fully sixty 

 times the resistance of silver. But between the 

 metals and liquids there is a yet wider difference. 

 If copper offer a resistance of i, then dilute acid 

 offers a resistance of i million ; a solution of 

 copper sulphate, of 17 millions ; and pure water 

 has the almost incredible resistance of 6700 

 million times that of copper. 



The law which the resistance of wires has 

 been found to follow is, that it is greater the 

 greater the length of the wire, and less the greater 

 its area of cross section. Or, in more precise 

 language, the resistance is proportional directly to 

 the length of a wire, and inversely to the area of 

 cross section or square of its diameter. Thus, we 

 should have as strong a signal by a telegraph wire 

 \ inch diameter, and nine miles long, as by one \ 

 inch diameter, and only one mile long. 



EFFECTS OF THE GALVANIC CURRENT. 



These may be classified as mechanical, magneti- 

 cal, physiological, heating, luminous, and chemical. 



The first two classes of effects will be described 

 under the distinct head of Electro-magnetism. We 

 have seen that the physiological were the first 

 observed effects of the current. They will be more 

 particularly noticed under Animal Electricity, to 

 which Galvani's experiment properly belongs. It 

 is remarkable that when the current passes through 

 the human body, nothing is felt except at the 

 beginning and end of the current flow. It is the 

 sudden change of tension that affects the nerves ; 

 and that is the reason why the frictional shock is 

 more painful than the galvanic. A battery of even 

 fifty Bunsen cells may be handled without much 

 inconvenience. Only this is so far owing to the 

 insulating nature of the skin. If we wet our hands 

 with salt water, the effect is greatly increased ; 

 and a very weak current will be felt if we let it 

 enter by a cut in the skin. 



The senses may be readily affected by the 

 current, especially those of taste and sight. If we 

 put a half-crown under, and a piece of zinc above 

 the tongue, a peculiar salt taste is perceived 

 whenever we make them touch. When the pieces 

 are put between the gums and the cheeks, a flash 

 of light is seen each time they are joined. The 

 nerves of hearing are not so quick, but with the 

 poles of a battery of thirty cells inserted in the 

 ears, it is said that a continuous noise is heard. 



Heating Effects. When a current passes through 

 a thin wire, the wire becomes heated, more or less 

 according to the strength of the current and the 

 resistance of the wire. A fine steel wire, which 

 offers great resistance, may be made white-hot, 

 melted, or even dissipated into vapour. Even 

 platinum, which cannot be fused by the heat of a 

 furnace, may be melted and volatilised by a battery 

 of thirty to forty Bunsens. 



A useful application of the heating power of 

 the current has been made to the firing of gun- 

 powder. If the wires from a battery communicate 

 with a fine steel wire embedded among gunpowder, 

 the passage of a current will heat the steel red-hot, 



