90 Dr. Faraday's Experimental Researches in Electricity : 



which was 21 inches long and 1*7 inch in diameter ; but one 

 end of this core was made smaller for the length of 1 inch, 

 being in that part only 1 inch in diameter. 



2644. On to this reduced part was fixed a hollow helix 

 consisting of 516 feet of fine covered copper wire: it was 

 3 inches long, 2 inches external diameter, and 1 inch internal 

 diameter: when in its place, 1 inch of the central space was 

 occupied by the reduced end of the electro-magnet core 

 which carried it ; and the magnet and helix were both placed 

 concentric with the metal cylinder above-mentioned, and at 

 such a distance that the latter, in its motion, would move 

 within the helix in the direction of its axis, approaching to 

 and receding from the electro-magnet in rapid or slow suc- 

 cession. The least and greatest distances of the moving cy- 

 linder from the magnet during the journey were one-eighth of 

 an inch and 2*2 inches. The object of course was to observe 

 any influence upon the experimental helix of fine wire which 

 the metal cylinders might exert, either whilst moving to or 

 from the magnet, or at different distances from it*. 



2645. The extremities of the experimental helix wire were 

 connected with a very delicate galvanometer, placed 18 or 20 

 feet from the machine, so as to be unaffected directly by the 

 electro- magnet ; but a commutator was interposed between 

 them. This commutator was moved by the wooden lever 

 (2643.), and as the electric currents which would arrive at it 

 from the experimental helix, in a complete cycle of motion or 

 to and fro action of the metal cylinder (2643.), would consist 

 of two contrary portions, so the office of this commutator was, 

 sometimes to take up these portions in succession and send 

 them on in one consistent current to the galvanometer, and at 

 other times to oppose them and to neutralize their result; and 

 therefore it was made adjustible, so as to change at any period 

 of the time or part of the motion. 



2646. With such an arrangement as this, it is known that, 

 however powerful the magnet, and however delicate the other 

 parts of the apparatus, no effect will be produced at the gal- 

 vanometer as long as the magnet does not change in force, or 

 in its action upon neighbouring bodies, or in its distance from, 

 or relation to, the experimental helix; but the introduction 

 of a piece of iron into the helix, or anj^thing else that can in- 

 fluence or be influenced by the magnet, can, or ought to, show 

 a corresponding influence upon the helix and galvanometer. 



* It is very probable that if the metals were made into cylinders shorter, 

 but of larger diameter than those described above, and used with a corre- 

 sponding wider helix, better results than those I have obtained would be 

 acquired. 



