6 Mr. Barlow on the Electro-magnetic conducting Power 



and observed variations average about a degree, and in one 

 case out of twenty exceed two degrees, and for this reason 

 I have only stated my result as an approximation; but admit- 

 ting the power to vary inversely as the length, the average error 

 amounts to nearly 20°, where the whole observed angle does 

 not amount to 16 ; and on this account I am again obliged to 

 employ a few lines to explain the manner in which these ex- 

 periments were conducted in order to trace that error to its 

 source. 



It is not easy to dispose of 838 feet of wire in such a way 

 that a needle shall not be disturbed by parts of it not intended to 

 act, and thereby the law of its action become misstated. I had 

 experienced this difficulty in some preliminary experiments, 

 which I attempted to make in a room with a much less length 

 of wire ; it is requisite therefore that I should explain the pre- 

 cautions taken to avoid this source of inaccuracy. On a grass 

 plat in my garden I formed a rectangular figure represented 

 in the diagram by G a V H, driving into the ground strong 

 stakes at G and H, and at a and b' short square open frames^ 





10 feet in periphery and 3 feet 6 inches high, (as shown by 

 abed, a! b' d d f ,) resembling the square frames or fences used 

 for young-planted trees. This rectangle was so planted that 

 G H, a b' were in the true magnetic meridian, and of course 

 its other two sides at right angles to the same. 



The wire being made fast at P, was turned round the prop 

 G, whence it passed to the frame E, about which it was made 

 to pass in 37 spiral threads from the bottom upwards; it then 

 passed from b to «', where it was made to descend in 37 spirals 

 from above downwards; it then proceeded from d 1 to H, 

 where it was turned round as at G to N. At P and N were 

 placed the two cups of mercury as described in the last ex- 

 eriments, which latter were also connected with the battery 

 , as already explained. 



E 



