ELECTRO-MAGNETISM. ' 



Lebaillif employs, instead of a single 

 wire, having, for instance, a length of 

 300 feet, five parallel wires, each sixty 

 feet long, the ends of which are stripped 

 of their silk coverings and united in a 

 bundle by being pressed together with 

 considerable force. In this way the 

 electric current which enters at one ex- 

 tremity is divided into five parts, and 

 made to flow, as it were, through five 

 separate channels. It is alleged, in 

 favour of this arrangement, that by thus 

 multiplying the channels of transmis- 

 sion, a proportionally larger quantity of 

 electricity is conveyed ; while the dimi- 

 nution of intensity arising from the 

 transmission of the same fractional part 

 of that current which passes through 

 one of the wires, along a great length of 

 wire, is avoided*. But experiments of 

 sufficient extent, and conducted with 

 sufficient care, appear to be wanting to 

 enable us to deduce any certain conclu- 

 sions with regard to this subject. The 

 only researches on this point, of which 

 we have been able to find an account, 

 are those of Dr. Kaerntz, who came to 

 the conclusion that the power of the 

 instrument to deflect the needle is 

 exactly in proportion to the number of 

 convolutions of the wire: six convolu- 

 tions giving six times the power of one 

 convolution t. But it would require a 

 much more extended investigation to 

 establish such a principle, and to fix the 

 limits of its operation. 



(123.) The advantage arising from 

 the employment of four needles instead 

 of two, in Mr. Lebaillifs instrument, 

 appears extremely dubious ; for it 

 should be recollected that if, on the one 

 hand, greater power is gained by the 

 action of the wire on the additional 

 needle, an equal addition is, on the 

 other hand, made to the weight that is to 

 be moved ; so that probably nothing is 

 thereby gained as to the motion indi- 

 cating that power. 



(124.) On account of the superior 

 conducting power of silver, wires of that 

 metal should be employed in preference 

 to those of copper; and they may then 

 be even as slender as the sixtieth of an 

 inch in diameter, which will allow of a 

 greater number of turns being included 

 in the same space. 



(125.) For the purpose of comparing 

 the intensities of two electrical currents, 

 an instrument has been contrived, which 



I * See Pouillet's Elmens du Physique Exncri- 

 loemale, tome i., p. G%. 



t I'liilobophical Magazine, vol. Ixii., p. 441. 



has been termed the Differential Galva- 

 nometer. Two wires of equal size are 

 twisted together, so as to form a com- 

 pound wire, which is coiled round the 

 compass needle, as in the instruments 

 already described ; and the four extre- 

 mities of the wires are immersed in four 

 cups filled with mercury. By this means 

 the two currents which are to be com- 

 pared with one another, may be trans- 

 mitted in opposite directions through- 

 out the whole extent of the coil. These 

 opposite currents, acting upon the needle 

 under precisely similar circumstances, 

 will, if they be equal, exactly counteract 

 each other, and the needle will remain 

 in equilibrio between the equal and con- 

 trary forces ; but if the currents be un- 

 equal in intensity, the needle will be 

 affected only by their difference, which 

 it will therefore indicate by its move- 

 ments. 



(126.) When, on the contrary, we 

 wish merely to ascertain the existence 

 and direction of an electric current, it 

 becomes an object to bring the current 

 as near as possible to the needle, so that 

 its action on the poles may be extremely 

 powerful. The following form has, with 

 this view, been given to the Galvano- 

 scope. The needle is suspended from 

 its centre by a fine thread, between four 

 vertical spiral coils, the centres of which 

 are brought very near to the poles of the 



Fig. 82. 



needle. The same current is made to 

 circulate through all the four spirals, 

 the turns of which are directed so as to 

 produce repulsion of the contiguous pole 



