110 



DOVE ON THK ELECTRICITY OF INDUCTION. 



fine iron borings enclosed in glass tubes, and piles were com- 

 posed of discs of sheet steel, of tinned and untinned sheet-iron ; 

 the discs were isolated by discs of paper; lastly, one cylinder 

 was composed of discs of tinned sheet-iron with interposed pieces 

 of silver coin : the diameter of these piles, consisting of several 

 hundred separate discs, was 9 lines. The prismatic rods were 

 composed of nickel, antimony, bismuth, zinc, lead, copper, iron, 

 brass, 1 8 inches long and 5 lines broad. Gold, silver, platina 

 and iridium were used in strips laid one upon the other. 



5. Although the same magnetizing spirals a b and c d, and the 

 same induction spirals a. /3 and y S, may be used with different 

 primary sources of electricity, yet it is preferable, yvhexi galvanic 

 currents are used and a strong action is required, to give greater 

 thickness to the connecting wire and a greater number of coils 

 to the collateral wire than when frictional electricity is employed, 

 and perfect insulation is then not so imperative as it is with the 

 latter icind of electricity. If however the magnetization of the 

 iron is eifected in a direct manner by approaching it to a steel 

 magnet, then the apparatus must be constructed in an essen- 

 tially different manner. In the following experiments I made 

 use of four different differential inductors ; the description of the 

 first three follows here, the last will be noticed in the sequel. 



2. Differential Inductor for galvanic and thermo-electricity. 



6. In the threads of two similarly cut screws of wood two spi- 

 rals of copper wire 2| lines thick*, insulated by means of shell-lac, 



* If it is required to use the apparatus here described 

 as an electro-magnetic macliine, in which induction takes 

 place by means of an electro-magnetized horseshoe of 

 iron, the arrangement depicted in the annexed figure 

 may be employed. A cylindrical bar of soft iron pp', 

 bent into the form of a horseshoe, is wound round at 

 the bend in the middle part of it by a thick spiral 

 wire of copper c d, which is prevented from coming into 

 contact with the iron by an intervening insulating sub- 

 stance. Upon the straight parallel limbs of the horse- 

 shoe, which are likewise covered witli an insulating sub- 

 stance, two straight cylindrical spirals of the same wire 

 a h and e I are placed, which being coiled in the same 

 direction as c d, form, when h is joined to c and d to e, 

 one continuous coil a bedel. The ends a / of these 

 two spirals proceed in a straight direction and paiallel on 

 the outer side of the limbs, so that they may not inter- 

 fere when the keeper is applied to the poles pp', nor pre- 

 vent the induction-spirals a /3 and e X, composed of long 

 thin wire, from being drawn over the coils of thick wire 

 of which the magnetizing spirals b a and e I are formed. 



