144 DOVE ON THE ELECTRICITY OF INDUCTION. 



tions, than an empty spiral wire without an iron nucleus, when 

 the galvanic current which magnetizes this iron ceases. The in- 

 crease of physiological action on breaking up this iron nucleus 

 into wires, and the greater degree of vividness of the sparks of 

 the induced current, as well as the increasing intensity of the 

 magnetism in a steel needle polarized by the current, are there- 

 fore due to an augmentation of the action already exerted by the 

 solid iron. The inducing action of the empty spiral which is 

 traversed by the momentary current produced by the discharge 

 of a Leyden jar is greater, as regards the physiological and elec- 

 troscopic actions of the secondary current, than when a solid 

 iron nucleus is contained in it ; it is less powerful however than 

 that which is produced by the insertion of a bundle of iron wire, 

 a longitudinally cut iron tube, or a solid rod of nickel. If the 

 bundle of wire is surrounded with an entire case, the bundle 

 which previously exerted an augmenting action now acts as a 

 solid rod, i.e. has a weakening effect. The heating effect of 

 the secondary current is on the contraiy diminished both by 

 solid iron and bundles of wire, indeed by iron in every shape, 

 as well as by unmagnetic metals : the capability of magnetizing 

 steel is increased by iron and nickel in every form, but it is di- 

 minished by solid rods of unmagnetic metals. 



e. If the connecting wire of the galvanic circuit or of the Ley- 

 den jar exerts an inducing action, not upon the secondary wire, 

 but upon its own parallel coils, this extra current exhibits to all 

 the tests that could be applied the same relations as the se- 

 condary current. 



f. The influence of conducting cases is caused by an electrical 

 current induced in them by the connecting wire, which can be 

 shown to exist in them, by connecting the edges of the longitu- 

 dinally cut cases by means of a galvanometer, or some other 

 kind of rheoscope. The same is true of the ends of enclosing 

 spiral wires, which, simply coiled, exhibit a current when their 

 ends are connected by the galvanometer, but on the contrary, 

 show no current when they are composed of a doubled wire, and 

 then closed by the galvanometer. Tubes and enclosing spirals 

 weaken the physiological action of the bundles of wire contained 

 within them, so much the more the better the substance con- 

 ducts of which they are composed. With solid iron rods the 

 surface acts as a conducting case enclosing an insulated bundle 



