DOVE ON THE ELECTRICITY OF INDUCTION. 155 



on becoming evanescent produces an electric current in a se- 

 condary wire, which can always be proved by its magnetizing 

 a steel needle. The polarity of this steel needle always remains 

 the same when a magnetizable metal is inserted into one of the 

 previously compensated spirals of the differential inductor, but 

 it is weaker when the magnetizable metal is in the form of a 

 soUd rod or a pile of discs than when it is a bundle of insu- 

 lated wires. The polarity of this needle is on the contrary re- 

 versed when the inserted metal is unmagnetic. In this case it 

 is in favour of the current produced by the empty spiral. 



In the electroscopic and physiological phaenomena of the cur- 

 rent induced by electro-magnetized iron and nickel, the remark- 

 able fact was established, that the less powerfully magnetic 

 nickel has an augmenting action, whilst the more powerfully 

 magnetic iron diminishes the effect, because the retarding elec- 

 tric cun-ents cannot be so readily formed in the badly con- 

 ducting nickel as in the better conducting iron ; i. e. in relation 

 to the electroscopic and physiological tests, solid iron acts as 

 a non-magnetic metal, whilst it acts as a magnetic metal as re- 

 gards the magnetization of the steel needle. Now, we may rea- 

 dily infer that the so-called non-magnetic metals have the same 

 action in relation to that property of the current which magnet- 

 izes steel, that iron has in relation to the electroscopic and phy- 

 siological properties, i. e. that they appear unmagnetic, because 

 the electric currents excited simultaneously with the magnet- 

 ism obscure the action of the magnetic polarity, but that they 

 really are not unmagnetic. It is therefore only necessary, in 

 order that the latter action should predominate, to prevent the 

 formation of electric currents, i. e. to break them up also into 

 wires, and then test the direction of the induced current by mag- 

 netizing a steel needle. If the current proceeds from the spiral 

 containing the bundle of wires, the metal is magnetic ; if it 

 proceeds on the contrary from the empty spiral, it is non-mag- 

 netic. 



65. For preliminary experiments brass was chosen. In the 

 form of a cylinder it diminished the intensity of the current from 

 its spiral, for the resulting current proceeded from the empty 

 spiral ; when the brass wires which were inserted had a certain 

 thickness, the equilibrium of the current was preserved ; when 

 thin, well-varnished brass wires were used, the current on the 



