222 The Astronomer Royal's Experiments on 



ones. by Coulomb in the French Memoirs for 1789 and other 

 years. It appeared to me that it might be desirable to make 

 experiments of a rather more extensive character, and to add 

 some measures of the magnetic effect of galvanic currents, both 

 directly by their immediate action, and indirectly by the amount 

 of magnetic power which they produce inductively in soft iron. 



For the measure of permanent magnetism I selected a bar 

 magnet 14 inches long, 1'4 inch broad, 0*35 inch thick; it has 

 not been touched by a magnet for several years, and is likely to 

 be in a state of very permanent magnetism. For the galvanic 

 currents a cylindrical coil was used 13*4 inches long, 1*4 inch 

 in external diameter, and about 0*9 inch in internal diameter; 

 it has, I believe, four layers of wire, each layer having 160 re- 

 volutions of the wire. The battery used with it consisted of 

 three cells, with sulphuric acid diluted in the proportion of 1 to 

 6; the plates were of zinc and graphite, each exposing on each 

 side about 8 square inches ; the circuit was always completed 

 about half an hour before the experiments were begun; and a 

 delicate galvanometer was placed in circuit, by which the steadi- 

 ness of the current was established. A core of iron 0*8 inch in 

 diameter and of the same length as the coil, removable at plea- 

 sure, fits well in the inside of the coil ; the iron is quite soft, 

 and can with ease be entirely freed from any subpermanent 

 magnetism. 



The first step in the experiment was to neutralize terrestrial 

 magnetism within the area of magnetic experiment. For this 

 purpose two powerful 2-feet magnets were placed below the 

 table on which the experiments were made, with their red or 

 north-seeking ends directed to the magnetic north, at a distance 

 (determined by trial) such that the experimental compass was 

 sensibly uninfluenced by terrestrial magnetism. It is possible 

 that some small residual magnetism was perceptible in the com- 

 parison with the feeble galvanic action ; but none could be cer- 

 tainly discovered in the other experiments. 



The compass used for register of the magnetic action is a 

 small and very lively pocket-compass, with needle 1*0 inch 

 long, not loaded with a card. The box of this compass is 

 circular; and when positions had been selected for the centre of 

 the compass (as will be mentioned), a circle somewhat larger 

 than the compass-box was described in pencil with each of those 

 positions for centre ; and the compass could then be planted 

 with its centre very accurately placed above the intended 

 point. 



The compass-positions were thus prepared : — Upon a sheet of 

 strong paper the plan of the magnet, 14 inches by 1*4 inch, 

 was laid down. On each side were drawn two parallel lines, of 



