48 Mr. R. Adie on the Electrical Cwrents induced 



gaged with the subject, I thought that the heating power of 

 the primary current on entering a bad conductor like bismuth 

 at A might be the cause of the galvanometer deflection ; but on 

 my return to the experiments, in consequence of the objection 

 made to them by Dr. Tyndall, I found I could not verify this 

 view by artificial heating at A. I therefore began to doubt if 

 the effects observed in the metallic cross had anything to do 

 with therrao-electrical currents ; and to try this point, I made 

 a cross of copper. The central joint E was effected by tying the 

 bars firmly together with cord, the surfaces in contact being 

 previously cleaned ; the other connexions with it were the same 

 as described in the diagram. In this arrangement the current 

 from the battery had a circuit through AED of copper only, and 

 yet there was generated in the other arms a current moving in 

 the direction BEC which deflected the galvanometer 38°; the 

 action being nearly the same as I had obtained from a bismuth 

 and antimony cross, soldered at their centre, but otherwise 

 under like conditions. An experiment was next made with the 

 copper cross soldered with tin at the centre E ; this produced a 

 deflection of 6°. A cross of copper was then prepared with the 

 metal at the centre as nearly homogeneous as possible ; from it 

 no permanent effect on the galvanometer could be obtained. 

 These three experiments show clearly that it is the nature of 

 the joint which the primary current has to cross at E that governs 

 the current induced in the outer pair of arms BE and CE; 

 and that it is not a thermo-electrical effect, is shown by the deflec- 

 tion of 38°, obtained when the joint consisted of two like pieces 

 of copper tied together. 



From crosses made of other metals the following indications 

 were obtained : — 



Deflection. 

 Copper cross centre joint homogeneous metal none, 



tin soldered at E . 6° 



tied with cord E . 38° 



Antimony-bismuth cross soldered at E, pri-~l ' o^o 

 mary current passed in the direction DEA j 



currentpassedin the direction AED 40° 



Lead cross tied with cord at E .... 64° 



Zinc ... ... .... 68° 



Antimony ... ... .... 70° 



Iron ... ... .... 71° 



Bismuth ... ... .... 76° 



Bismuth cross soldered at E with tin . . 46° 



In these experiments a cross composed of two bars of bismuth 

 tied at E, shows a much more energetic current than the anti- 



