2 Mr. Barlow on the Electro-magnetic conducting Poxver 



dertook the investigation of several of these laws, such as the 

 general character and direction of the magnetic force, the law 

 of its power as depending on the distance and relative position 

 of the compass and conductor, and on the law as depending 

 on the length of the conductor immediately acting on the 

 compass, and some others. My paper on this subject was read 

 at a meeting of the Royal Society in May 1823, and was af- 

 terwards published in the second edition of my Essay on Mag- 

 netic Attractions. 



In the experiments above referred to, made with a view of 

 determining the law as depending on the length of the wire, the 

 whole length of the conductor remained nearly the same, and 

 the question was only respecting the change of angle depend- 

 ing upon the length of that part of the conductor which acted 

 directly on the needle; but in 1824, in continuation of these 

 inquiries, I undertook to investigate the laws of the conducting 

 power, or rather, perhaps, the laws of electro-magnetic inten- 

 sity, as depending upon the actual length and diameters of 

 the conducting wires. These experiments were published 

 early in 1825 in the Edinburgh Philosophical Journal. 



It had been ascertained at that time, that a diminution of 

 deflecting power exhibited itself on the needle by any consi- 

 derable lengthening of the conducting wire, and my first ob- 

 ject was, if possible, to determine the law of this diminution, 

 as also what the law was in wires of different diameters. The 

 same experiments which I had proposed for this purpose would 

 likewise, I imagined, determine another important question, 

 namely, whether the effect was produced by the transmission 

 of one fluid from the positive to the negative side of the battery; 

 or whether two fluids rushed simultaneously from each pole ; 

 or lastly, whether the effect was due to the transmission of 

 any fluid whatever. My reasoning stood thus : If the effect 

 is due to a single fluid passing from the positive to the nega- 

 tive side of the battery, and the diminishing effect, by length- 

 ening the wire, is due to a dissipation of the fluid in its pro- 

 gress, a compass placed near the positive pole ought to be 

 more deflected than one near the negative pole ; or if two 

 fluids rush simultaneously from both poles, the compasses at 

 these poles ought to be more strongly deflected than one placed 

 in the centre, supposing, as I had done, that the diminished 

 effect was the result of dissipation. I possessed at that time a 

 powerful galvanic battery, consisting of 20 pairs of zinc and 

 copper plates, 10 inches square, arranged after the manner of 

 Dr. Hare's calorimotor ; those of the latter being in connection 

 with the copper lining of the battery, amounting to 16 square 

 feet; so that I had about 30 square feet of zinc, and 46 square 



