138 DOVE ON THE ELECTRICITY OF INDUCTION. 



much less mass of metal, the tube and the bundle being alike in 

 external circumference. A simple method of testing whether a 

 metallic bar placed within one of the tubes destroj'S the physio- 

 logical equilibrium of the current in the secondary spirals by 

 weakening the action of its spiral, is that of placing brass wires 

 into the other empty tube, a certain number of which must even- 

 tually restore the equilibrium which had been disturbed. 



47. Forged iron, soft and hard steel, white and gray pig iron 

 in the form of solid cylinders and prismatic rods, and also in 

 the form of entire tubes, as gun-barrels and welded tubes of sheet 

 iron, all weaken the physiological action of the induced cur- 

 rent. The same is the case with piles of discs of steel, of forged 

 iron and of tinned sheet iron, both when they are arranged 

 with insulating and conducting discs between them. The cur- 

 rent produced by forged iron and steel, tested by the conden- 

 ser and the resinous figures, proceeded from the empty spiral. 

 The weakening influence of forged iron, steel and pig iron, is 

 however different ; for with two opposing cylinders of differ- 

 ent kinds of iron in the compensated spirals, vibratory mo- 

 tions are always observed on an insulated preparation of the 

 frog. 



48. The physiological action, on the contrary, is increased 

 by longitudinally cut gun-barrels, and particularly by well insu- 

 lated bundles of iron wire. A shock from the similarly connected 

 secondary spirals that was perceptible in the joints of the hand, 

 extended to the middle of the upper arm upon the insertion of 

 two such bundles of wire, but it was so weakened by the inser- 

 tion of two cylinders of wrought iron, that it could only be felt 

 in the extremities of the hand. The cun-ent tested by the con- 

 denser and the resinous figures, with compensated spirals, pro- 

 ceeded from that spiral in which the bundle of wire was placed. 

 Here then there is a marked distinction between the inducing 

 action of iron depending upon the manner in which it is mag- 

 netized, whether by the current of a galvanic battery, or by that 

 produced by the discharge of a Leyden jar. The inducing ac- 

 tion of the spiral-shaped connecting wire of a galvanic battery 

 upon a secondary wire is increased when iron in any shape is 

 inserted into it, whilst the connecting wire of a Leyden jar has a 

 less powerful inducing action upon a secondary wire when a 

 solid iron rod is inserted in it, than when it is empty; and 



