Prof. Thomson on Rapid Signalling by the Electric Telegraph. 63 
of the dimensions of the Atlantic cable, either actually submerged or 
placed: in perfectly similar inductive circumstances. 
Il. Method for telegraphing through submarine or subterranean 
lines of not more than 500 miles length. 
The plan which I have proposed to describe for rapid signalling 
through shorter wires, has one characteristic in common with the 
plan I have already suggested for the Atlantic telegraph ; namely, 
that of using different strengths of current for different signals. 
But in lines of less than 500 miles, condensed pulses, such as have 
been described, may be made to follow one another more rapidly 
than to admit of being read off by an observer watching the image of 
a scale in a suspended mirror; and a new plan of receiving and 
recording the indications becomes necessary. 
Of various plans which I have considered, the following seems 
most likely to prove convenient in practice. 
Several small steel magnets (perhaps each about half an inch 
long) are suspended horizontally by fine threads or wires at different 
positions in the neighbourhood of a coil of which one end is con- 
nected with the line wire and the other with the earth. Each of 
these magnets is held in a position deflected from the magnetic 
meridian by two stops on which its ends press; and two other small 
stops of platinum wire are arranged to prevent it from turning 
through more than a very small angle when actuated by any de- 
flecting force making it leave the first position, When a current 
passing through the coil produces this effect on any one of these 
magnets, it immediately strikes the last-mentioned stops, and so 
completes a circuit through a local battery and makes a mark on pre- 
pared electro-chemical paper. For each suspended magnet there is 
a separate style, but of course one battery is sufficient for the whole 
printing process. One set of the different suspended magnets are so 
adjusted, that a current in one direction of any strength falling short 
of a certain limit makes only one of them move; that a current in 
the same direction, of strength exceeding this limit but falling short 
of another limit, moves another also of the suspended magnets ; and 
so on for a succession of different limits of strength of current in one 
direction, The remaining set of suspended magnets are adjusted to 
move with different strengths of current in the other direction 
through the coil. Without experience it is impossible to say how 
many gradations of strength could be conveniently arranged to be 
thus distinguished unmistakeably. I have no doubt, however, that 
very moderate appli.ations of electric resources would give at least 
three different strengths of current in each direction, which could 
with ease and certainty be distinguished from one another by the test 
which the suspended magnets afford. Thus, a signal of six varieties 
—one letter of an alphabet of six—could be recorded by almost in- 
stantaneous movements of six suspended magnets, making one, two 
or three marks by one set of three styles, or one, two or three marks 
by another set of three styles, placed all six beside one another, 
ressing on a slip of electro-chemical paper drawn by clockwork, as 
im the Morse instrument. 
