DOVE ON THE ELECTRICITY OF INDUCTION. l79 



Ijroken; if, on the contrary, 7) is connected with 12) and 15) 

 Avith 8) by means of wires, then when that spring touches the 

 wood the previously existing secondary connexion I and II is 

 broken. 



It must be however borne in mind, that this case, as well as 

 that of chemical decomposition, which will be considered directly, 

 is not perfectly comparable with the experimental arrangement 

 in the physiological experiments ; for as the body presents a 

 considerable resistance to conduction, its influence upon the 

 pi'imary current could be disregarded as long as it formed a 

 secondary connexion with u closed. This is by no means the 

 case when, as here with u closed, either I and II or II and III 

 forms a perfect metallic secondary connexion, in which case the 

 galvanometer shows that a great part of the primary current 

 takes this route. Now we can always picture to oiu'selves the 

 extra current excited in the extra spiral under the form of a 

 greater resistance, which this spiral opposes to the primary cur- 

 rent^ produced by the keeper. The insertion of iron increases 

 this resistance, and in this case a greater portion of p will pass 

 through the secondary connexion I and II than when no iron is 

 present in the extra spiral ; and indeed the spark is then much 

 more brilliant, namely, that on the cylinder w^, whilst that at it 

 on the cylinder iv^ is nearly extinguished. AVith the connexion 

 II and III the augmentation of the spark in the secondary con- 

 nexion at iv^ is not remarkable, inasmuch as E is there increased. 

 As with the adopted arrangement of the apparatus the spark on 

 breaking connexion at u on the cylinder zi'g, and that between I 

 and III or II and III on the cylinder lo^, appear directly side by 

 side, the growing intensity of the one corresponding to the de- 

 creasing intensity of the other when iron is inserted into the 

 extra spiral, presents a very instructive spectacle. 



3. Chemical decomposition. 

 84. If the current is retained constantly in the same direction 

 by means of the forked springs y y, and if the voltameter is to be 

 introduced immediately into the circuit of this current, but not 

 as a secondary connexion, then the connexion will be ditFei'cnt 

 when an extra spii-al is inserted than when that is not done. 

 If the wires I and II proceeding from 4) and 8) lead to the vol- 

 tameter, this then forms the connexion between the two cylin- 

 ders tt;, and w^ by means of the stands C and D, as only one 



