6 Prof. J. C. Poggendorff on the Induction Apparatus 



as, or more so than, one of equal weight prepared in the most 

 careful manner by means of strings and individually insulated 

 bars. Two of the cores which I prepared are a little longer than 

 the induction coil, 6 inches long and 13 ounces in weight. A 

 third is three times as long, i. e. 18 inches. Each of them con- 

 tains 4200 wires of the given dimensions. 



I convinced myself, however, that this number may be dimi- 

 nished without injury to the action of the coil, in which case, of 

 course, the primary coil will remain partly unoccupied. In fact, 

 I found that a hollow core of the above-mentioned wires, which 

 contained an empty cylindrical space three-quarters of an inch in 

 diameter, and which weighed only half as much as the massive 

 ones, was just as effective as the latter. It surprised me to 

 observe that when a massive bar of soft iron was inti'oduced into 

 the hollow axis of this core, the action of the induction coil was 

 not thereby diminished ; whereas the same soft iron bar intro- 

 duced alone, in absence of the hollow core, unmistakeably dimi- 

 nished the action which the primary coil itself exercised upon 

 the induction coil. This fact is evidently analogous to one ob- 

 served several years ago by Magnus, viz. that a massive bar of 

 iron, when introduced into a slit tin tube, weakened but little 

 the action of the latter*. 



Practically the soft iron core is of great use, theoretically it 

 causes much complication. If it were dispensed with on this ac- 

 count, it would be necessary to give the apparatus a different foi-m 

 in order not to diminish its action. It would be necessary to in- 

 crease the diameter of the inducing coil, and to arrange the 

 induction wire half inside it and half outside it, in order to make 

 use of both sides of the inducing wire. The soft iron core has 

 a strengthening influence, because, by a kind of reflexion, the 

 inner side of the inducing coil is made to act externally. 



Current-Breaker. 



For bx'eaking the current I employed as usual a Neef's, or 

 rather AVagnei's hammer, of which I possess two. One is, on 

 the whole, constructed in the usual manner, i. e. a fixed pin 

 descends, and against it, from below, beats a spring tongiie with 

 a small plate fixed to it. According to an improvement intro- 

 duced by Kiess, there is a second pin imderneath the tongue, in 

 order to make use of the downward vibrations for closing the 

 circuit ; so that the primary current can not only be doubly 

 broken, but also reversedt. These effects can be accomplished 

 in my instrument by means of a single galvanic battery in con- 

 sequence of the two wires around the primary coil, the combiua- 



* Poggendoi-ff's Annalen, 1839, vol. xlviii. p. 105. 

 t Il)i(l. vol. xci. p. 290. 



