518 



Tlie Rev. T. R. Robinson's Experimental Researches on the 

 Table XI. 



The formula gives more power to the spires of C, less to those of the zones 

 D—C and E — D than is observed: if, in fact, we determine what parts of the 

 eifect of E belong to them, we find for 



C eflect = 10-4967 = 9 x 1-1663. 

 D-C „ 9-5033 = 10x0-9503. 

 E~D „ 9-5080 = 10x0-9508. 

 It appears ^.M'oiai/t', from Table i., that the two or three innermost spires act 

 much more powerful!}' than the rest ; and therefore it seems that, from them 

 to a considerable distance, the exciting force of the others is constant. The dis- 

 crepancy between the computed and observed ju is far too great to be caused 

 by error of observation ; for again taking D as the standard, the theoretic fx will 

 give L' 48 greater for C, and 61 less for E, than the true values, while the 

 probable errors of the latter are only 2-04 and 1-99. 



ACTION OF HELICES. 



Here also the discrepancy between theory and observation is considerable, 

 independent of the interference of induction. This is shown, even without mea- 

 sures, by some striking facts ; for instance, the helices {F) being placed above the 

 polar surfaces of the cylinders set to 4-1, but separated by plates of zinc -^ thick, 

 and e.xcited to have i^ = 552, the magnetism produced was scarcely sensible 

 when a keeper was applied across the cylinder, immediately below them ; I had 

 no means of measuring it, but its attraction was not more than a pound or two. 

 Had the helices been on tlie cylinders, L would be 850. If one of these 

 helices be placed in the same way above one polar surface, there is scarcely any 

 attractive power developed at the other : in this case, however, as the magnetic 

 circuit is incomplete, the force is much less ; but I expected to find 30 or 40 

 pounds at least. Similar results were obtained with a magnet, the upper two 



