158 DR. FARADAY’S EXPERLMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIX.) 
placed, the design still remains recorded. When the designs are to be preserved in 
blue only, the gum may be dispensed with and the red ferroprussiate solution only be 
used. 
3237 . It must be well understood that these forms give no indication by their 
appearance of the relative strength of the magnetic force at different places, inas- 
much as the appearance of the lines depends greatly upon the quantity of filings and 
the amount of tapping; but the direction and forms of the lines are well given, 
and these indicate, in a considerable degree, the direction in which the forces in- 
crease and diminish. 
3238. Plate IX. fig. 1, shows the forms assumed about a bar-magnet. On 
using a little electro-magnet and varying the strength of the current passed through 
it, I could not find that a variation in the strength of the magnet produeed any 
alteration in the forms of the lines of force external to it. Fig. 2 shows the lines 
over a pole, and fig. 3 those between contrary poles. The latter accord with the 
magnetic curves, as determined and described by Dr. Roget and others, with the 
assumption of the poles as centres of force. The difference between them and those 
belonging to a continuous magnet, shown in fig. 1, is evident. Figs. 4, 5 show the 
lines produced by short magnets. In the latter case the magnet was a steel disc 
about one inch in diameter and 0-05 in thickness. Fig. 6 shows the result when a 
bar- magnet is broken in half, but not separated. Fig. 7 shows the development of 
the lines externally at the two new ends as the halves are more and more separated 
(3231.). Figs. 8, 9 and 10 present the results, with the two halves or new magnets 
in different positions. Figs. 11, 12, 13 and 14 show the results with disc magnets. 
Fig. 15 shows the condition of a system of magnetic forces when it is inclosed by a 
larger one, and is contrary to it. Fig. 16 shows the coalescence of the lines of force 
(3226.) when the magnets are so placed that the polarities are in aecordance. 
3239. Fig. 17 exhibits the lines of force round a vertical wire carrying a current of 
electricity. Whether the wire was thick or thin appeared to make no difference as 
to the intensity of the forces, the current remaining the same. Fig. 18 represents the 
lines round two like currents when within mutual influence. Fig. 19 shows the 
result when a third current is introduced in the contrary direction. Fig. 20 presents 
the transition to a helix of three convolutions. Fig. 21 indicates the direction of the 
lines within and outside the end of a cylindrical helix, on a plane through its axis. 
Fig. 22 presents the effect when a very small soft iron core is within the helix. 
3240. Figs. 23 and 24 give an experimental illustration of the principles which I 
have adopted in relation to atmospheric magnetism and the general cause of the 
daily variations, &c. (2864. 2917.)« A hemisphere of pure nickel presented to me by 
Dr. Percy, was supported with its flat face uppermost, and a large ring arranged 
round it to carry paper, which, resting both on the ring and the nickel, could then 
have iron filings sprinkled and arranged in form on it. The end of a bar-magnet in 
the same horizontal plane was adjusted about 2 inches from the nickel, and thus the 
