d 72 PHYSICS. 
paper on which the signs are to be made pass under a roller immediately 
above this point, being unwound from a coil, J, and worked by clock-work, 
as in Steinheil’s telegraph. When the horse-shoe becomes magnetic, it 
attracts the plate above it, by which means that extremity of the lever is 
depressed. The other end being elevated causes the steel point to strike 
into the strip of paper. As this strip is constantly moving under the 
above-mentioned roller, a sudden closing and opening of the circuit will 
produce a point on the paper ; if the circuit be kept closed for an appreciable 
period, the point being pressed all the time against the paper, a line will be 
made. Thus by the combination of dots and lines, a series of symbols 
answering to the alphabet will be produced. 
The most important laws of the magnetic action of the galvanic current, 
as worked out after numerous experiments by Jacobi and Lenz, are the 
following: 1. The amount of magnetism is proportional, other things 
being equal, to the strength of the galvanic current employed; 2. The 
thickness of the wire of the coil exerts no influence on the strength of the 
current’; 3. Neither does the diameter of the coil, if the iron projects far 
enough from it. It is thus all the same whether some of the windings 
are carried immediately about the middle of the iron, as in pl. 22, fig. 66, 
or at some distance, as in fig. 67. 4. The combined action of all the windings 
is equal to the sum of the actions of the single turns; 5. The magnetism of 
iron bars or rods of equal length, other circumstances being equal. is 
proportional to their diameters. 
As the galvanic current exercises magnetic influence, so on the other 
hand the magnet acts on the galvanic current. The influence of terrestrial 
magnetism is especially interesting in this respect. To detect this it is 
necessary to impart to the current a high degree of mobility, for which the 
apparatus of Ampére. represented in pl. 20, fig. 83, is especially calculated. 
Here ¢ and v are two brass pillars fixed in a board, and carrying horizontal 
arms above, which appear to be in contact, but in reality are separated by 
some non-conducting substance ; at the extremities of these arms are the 
two small cups, z and y, standing one above the other. When the feet of 
the pillars are brought into communication with the poles of a working 
battery, one cup becomes positively, the other negatively electrified. The 
contrivance represented in jig. 84 is intended for more readily breaking 
the connexion with the feet of the pillars and restoring it in the opposite 
direction. Here *,7’, are two grooves in a board, several lines thick ; v and 
v', t and ¢’, four holes which are connected in pairs by copper strips, namely, 
v with v' by Ul, t with t by mm. Where the strips cross each other they are 
separated by non-conductors. All the grooves and holes are varnished and 
filled with mercury. If 7 be connected with v, r’ with ¢, and the positive 
electrode dipped in the groove r, and the negative in 7’, the electricity is 
distributed in the wire v, from ¢ to ¢’, and the metal strips, b’ and 3, 
connected with v’ and t, become, the former positive, the latter negative. 
On the other hand, if 7 be connected with t’, and 7’ with v’, then b will be 
positive and b' negative. If both strips are connected by a wire, as in the 
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