DOVE ON THE ELECTRICITY OF INDUCTION, 125 



been previously tested when these were connected. The currents 

 are also simultaneously interrupted, as the notches leave the 

 mercury at the same moment. If different rods of iron or bun- 

 dles of wire are placed in the spirals, the sparks at each inter- 

 ruption, which before were alike, will be different. In this man- 

 ner it can be distinctly seen that a bundle of wire, which had 

 been previously completely compensated by a bar of soft iron as 

 regards the galvanometer, produces after separation a much 

 more vivid spark. 



27. In the same manner the heating effects can be mea- 

 sured by two electric thermometers, by which each of the sepa- 

 rated induction-spirals is closed, and in like manner the chemi- 

 cal decomposition when connexion is made by two voltameters. 

 With respect to these however no measurements have been 

 made ; it has only been ascertained that the heating effects, as 

 ■well as those of chemical decomposition of the empty spirals, 

 are very much increased by placing iron rods and bundles of 

 wires within them. 



4. Experiments with iron tubes. 



28. In some former experiments*, I had shown that an elec- 

 tro-magnetic spiral, which surrounded an iron tube of the dimen- 

 sions of a gun-barrel, was not capable of magnetizing an iron 

 cylinder placed w ithin the tube ; and vice versa, that a moveable 

 magnet, or a fixed electro-magnet placed within this tube, did 

 not excite any phjenomena of induction in the spiral which sur- 

 rounded itf. It follows therefore, that, with relation to the phai- 

 nomena which we are here examining, bundles of wire inclosed 

 within gun-barrels cannot increase the action of these ; for they 

 are as much protected by their iron case from the magnetizing 

 action of the spiral which closes the circuit, as their inducing 

 power itself is incapable of affecting the spiral of thin wire. 

 Sturgeon had previously made the observation, that wires in- 



* Bullet, de I'Acad. de St. Petersh. viii. ii. p. 20, and Repert. i. p. 276. 

 t Analogous phsenomena may be observed with hollow iron tubes which sur- 

 round straight steel magnets, and which may be considered as keepers connect- 

 j ing the opposite poles over the whole periphery of the magnet. A steel mag- 

 j net fitting tightly itito a hollow iron cylinder having the dimensions of a gun- 

 I barrel, or still thicker in metal, exhibits no action on its external surface. 

 ' Placed in a spiral, it scarcely induces any current; suspended by silk, both ends 

 are attracted by both poles of a magnet held near it ; it does not rotate when 

 1 exposed to the action of a rotating disc of copper ; it is therefore slill more 

 I neutralized than a horseshoe magnet is by a straight keeper, for this rotates 

 ■lightly under these circumstances. 



