J^lectro-magnetic Experiments. 73 



and a hole passing through it obliquely, through which the wire 

 went. The silk envelopes and the varnish seemed not only to 

 insulate the coils, but, when the varnish got fully dry, to unite 

 the whole compound covering into one solid mass. As the ends 

 were now to be soldered together, advantage was taken of the 

 opportunity to determine what effect the wire has in developing 

 the magnetic fluid, according to the distance from the extremi- 

 ties at which it is coiled on the iron. The ends of the three 

 tiers were soldered in succession to main conducting wires, each 

 one-quarter of an inch in diameter, and the effect each time was 

 observed by means of a compass needle. There were several 

 galvanic batteries employed, each consisting of a single pair of 

 plates, all of the same dimensions, and charged each time with 

 acid of the same strength. The needle employed was that of a 

 common theodolite, hung on agate, on a circle of about 5 inches 

 diameter, and graduated to single degrees. 



In each experiment the electro magnet was placed with its 

 poles pointing to the zenith, and in the plane of the prism ver- 

 tical ; and the compass being placed with its centre in the same 

 plane, was moved due westward, when it was shifted. One part 

 of sulphuric acid was mixed with 9 of nitric, and 1 part of this 

 nitro-sulphuric acid to 38 of water formed the acid employed. 



Experiment 1. — The first tier of coils was soldered into con- 

 nexion, the centre of the needle was placed at the distance of 

 4 feet from the centre of the neighbouring pole of the magnet, 

 and, on the battery being charged, the deflection was 14°. The 

 compass centre was then quickly shifted to the distance of 6 (eet, 

 and deflection was 7°. 



Experiment 2. — The second tier was then soldered in, and 

 on a fresh battery being charged, distance 6 feet, deflection was 

 8° ; which, on shifting the needle centre to 8 feet distance, di- 

 minished to 2° 5(y. 



Experiment 3. — The arch tier was now lastly soldered in, a 

 fresh battery charged, distance 8 feet, and deflection was 3" 45' ; 

 which, on increasing the distance to 10 feet, diminished to 1°. 

 This last deflection, though small, was very decided and well 

 defined. 



The following experiment was next made, to ascertain dif- 

 ferent relative effects of different strengths of acid. The electro- 

 magnet and the needle were disposed as before, the constant dis- 



