524 The Rev. T. R. Robinson's Experimental Researches, ^x. 



n 1 . / // S' p" ■}■ S" p R-^p-\-p" 



consecutive -f : collateral -^ •.: s +s : -^ 77-i x —rrr' 



P+P R+P-P- 



p'-\-p" 

 If, as is usual, s'=s", p' = p", the ratio becomes 



^■>^/.:'2R + p':R+'ip'. 



"When p is greater than R the collateral arrangement is best, but then it 

 must be observed, the expenditure of zinc and acid is twice as great as in the 

 ordinary arrangement. 



In these experiments the cylinders were entirely covered with spires; when 

 this is not the case, and particularly when the helices are at a distance from the 

 polar surfaces, the action is diminished, because it is imperfectly transmitted by 

 induction. Such observations seem calculated to advance our knowledge re- 

 specting that power, and I hope soon to submit to the Academy those which I 

 have made with this view ; and some respecting electro-magnets of hard steel 

 and cast iron. 



