476 ELECTRO-MAGNETIC EXPERIMENTS AND OBSERVATIONS. 



Similar experiments were repeatedly made with the zinc 

 west and east ; but in both those cases, the effects of the ver- 

 tical portions of the limbs were perfectly similar to those here 

 given under II., or Zinc South ; so that the same table will re- 

 present the action of the four sides of the vertical limbs in 

 three positions of the zinc. When the compass is carried 

 round one of these vertical limbs of the connecting piece, the 

 needle makes one revolution on its centre. 



We next examined the effect of the horizontal part of the 

 rectangular connecting piece on the magnetic needle. 



I. Zinc North. 



1 . Needle above the horizontal part deflected 90° E. 



2. - below - - 90° W. 



II. Zinc South. 



1. Needle above the horizontal part deflected 90° W. 



2. - below 90° E. 



III. Zinc West. 



1. Needle above shewed no deflection, but vigorous action. 



2. - below had its poles inverted. 



IV. Zinc East. 



1. Needle above had its poles inverted. 



2. - below shewed no deflection, but vigorous action. 



When the needle was applied to the two perpendicular sur- 

 faces of the horizontal part, there was a difference of deflec- 

 tion, as it approached either the upper, or the lower surface, 

 partaking of the effect of that surface to which the compass 

 was approximated. The action of the upper and under sur- 

 faces of the rectangular conducting piece, when the zinc was 

 north or south, did not differ from that of the simple connec- 

 ting wire in Fig. 2. ; but the breadth of these surfaces de- 

 veloped 



