I 



and its Effects. 5 



form, except that they contain two insulated copper wires which 

 can be used either singly or combined, side by side or one after 

 the other. 



The wire is 1 millim. thick, and each of its halves about 100 

 feet long. The tube upon which the wire is coiled in four layers 

 is of mill-board in two instruments, and of glass in the third, 

 and has an internal diameter of li inch. 



In some cases I have also used a fourth primary coil with wire 

 only 0-67 millim. in diameter, and nearly 400 feet long ; this 

 was coiled on a mill-board tube. 



It may be here mentioned, that, under otherwise equal cir- 

 cumstances, the inducing action of a current, just as its magnetic 

 action, is a product of its intensity and length ; hence a short, 

 thick wire and great intensity of current may sometimes be ad- 

 vantageously replaced by a long, thin v/ire, and small intensity 

 of current*. The last is preferable when the action requires to 

 be long sustained, because the voltaic battery is less attacked ; 

 on the other hand, however, the inner induction current, the so- 

 called extra current, increases in intensity, and leads to disad- 

 vantages, of which more hereafter. 



Soft Iron Core. 



Usually this is formed of pretty thick wires, or we may almost 

 say, small bars, held together by a common cap, but yet sepa- 

 rated from each other by some insulating medium. 



In some cases I have employed such a collection of bars, but 

 oftener I used cores of much thinner wire ; on the one hand, 

 because this thinness must tend to increase the action, and on 

 the other hand, because I was enabled to construct such cores for 

 myself. The wire was only 0-25 millim. in diameter. It was 

 heated to redness, cut into pieces of convenient length, bound in 

 a bundle with silk thread, and lastly, surrounded with a paper 

 covering. 



I found it unnecessary to cover the wires with a varnish, 

 partly because the stratum of protoxide which forms on its sur- 

 face, when heated, is comparatively a bad conductor ; partly 

 because the irregularity of the spaces between the wires, none of 

 which remain straight, is more than sufficient to destroy the 

 pernicious continuity in the direction of the periphery of the core. 



If care be taken not to diminish the interstices too much by 

 unnecessary tightening, such a soft iron core will be as effective 



* The voltaic battery and the volume of the inducing wre of an induc- 

 tion coil being given, the maxhnuni inductive effect is obtained when the 

 resistance of the inducing wire is equal to the residual resistance in the 

 battery ; the action of the extra cunent, however, is here neglected. Vide 

 Poggendorff 's Annalen, vol. Iv. p. 45. 



