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f light is thrown from a lamp upon the mirror； and reflected by it upon a white screen or scale a few feet distant； where it forms a bright spot of light。
When there is no current on the instrument； the spot of light remains stationary at the zero position on the screen； but the instant a current traverses the long wire of the coil； the suspended magnets twist themselves horizontally out of their former position； the mirror is of course inclined with them； and the beam of light is deflected along the screen to one side or the other； according to the nature of the current。 If a POSITIVE currentthat is to say； a current from the copper pole of the batterygives a deflection to the RIGHT of zero； a NEGATIVE current； or a current from the zinc pole of the battery； will give a deflection to the left of zero； and VICE VERSA。
The air in the little chamber surrounding the mirror is compressed at will； so as to act like a cushion； and 'deaden' the movements of the mirror。 The needle is thus prevented from idly swinging about at each deflection； and the separate signals are rendered abrupt and 'dead beat；' as it is called。
At a receiving station the current coming in from the cable has simply to be passed through the coil of the 'speaker' before it is sent into the ground； and the wandering light spot on the screen faithfully represents all its variations to the clerk； who； looking on； interprets these； and cries out the message word by word。
The small weight of the mirror and magnets which form the moving part of this instrument； and the range to which the minute motions of the mirror can be magnified on the screen by the reflected beam of light； which acts as a long impalpable hand or pointer； render the mirror galvanometer marvellously sensitive to the current； especially when compared with other forms of receiving instruments。 Messages have been sent from England to America through one Atlantic cable and back again to England through another； and there received on the mirror galvanometer； the electric current used being that from a toy battery made out of a lady's silver thimble； a grain of zinc； and a drop of acidulated water。
The practical advantage of this extreme delicacy is； that the signal waves of the current may follow each other so closely as almost entirely to coalesce； leaving only a very slight rise and fall of their crests； like ripples on the surface of a flowing stream； and yet the light spot will respond to each。 The main flow of the current will of course shift the zero of the spot； but over and above this change of place the spot will follow the momentary fluctuations of the current which form the individual signals of the message。 What with this shifting of the zero and the very slight rise and fall in the current produced by rapid signalling； the ordinary land line instruments are quite unserviceable for work upon long cables。
The mirror instrument has this drawback； however it does not 'record' the message。 There is a great practical advantage in a receiving instrument which records its messages； errors are avoided and time saved。 It was to supply such a desideratum for cable work that Sir William Thomson invented the siphon recorder； his second important contribution to the province of practical telegraphy。 He aimed at giving a GRAPHIC representation of the varying strength of the current； just as the mirror galvanometer gives a visual one。 The difficulty of producing such a recorder was； as he himself says； due to a difficulty in obtaining marks from a very light body in rapid motion； without impeding that motion。 The moving body must be quite free to follow the undulations of the current； and at the same time must record its motions by some indelible mark。 As early as 1859； Sir William sent out to the Red Sea cable a piece of apparatus with this intent。 The marker consisted of a light platinum wire； constantly emitting sparks from a Rhumkorff coil； so as to perforate a line on a strip of moving paper； and it was so connected to the movable needle of a species of galvanometer as to imitate the motions of the needle。 But before it reached the Red Sea the cable had broken down； and the instrument was returned dismantled； to be superseded at length by the siphon recorder； in which the marking point is a fine glass siphon emitting ink； and the moving body a light coil of wire hung between the poles of a magnet。
The principle of the siphon recorder is exactly the inverse of the mirror galvanometer。 In the latter we have a small magnet suspended in the centre of a large coil of wirethe wire enclosing the magnet； which is free to rotate round its own axis。 In the former we have a small coil suspended between the poles of a large magnetthe magnet enclosing the coil； which is also free to rotate round its own axis。 When a current passes through this coil； so suspended in the highly magnetic space between the poles of the magnet； the coil itself experiences a mechanical force； causing it to take up a particular position； which varies with the nature of the current； and the siphon which is attached to it faithfully figures its motion on the running paper。
The point of the siphon does not touch the paper； although it is very close。 It would impede the motion of the coil if it did。 But the 'capillary attraction' of so fine a tube will not permit the ink to flow freely of itself； so the inventor； true to his instincts； again called in the aid of electricity； and electrified the ink。 The siphon and reservoir are together supported by an EBONITE bracket； separate from the rest of the instrument； and INSULATED from it； that is to say； electricity cannot escape from them to the instrument。 The ink may； therefore； be electrified to an exalted state； or high POTENTIAL as it is called； while the body of the instrument； including the paper and metal writing…tablet； are in connection with the earth； and at low potential； or none at all； for the potential of the earth is in general taken as zero。
The ink； for example； is like a highly…charged thunder…cloud supported over the earth's surface。 Now the tendency of a charged body is to move from a place of higher to a place of lower potential； and consequently the ink tends to flow downwards to the writing…tablet。 The only avenue of escape for it is by the fine glass siphon； and through this it rushes accordingly and discharges itself in a rain upon the paper。 The natural repulsion between its like electrified particles causes the shower to issue in spray。 As the paper moves over the pulleys a delicate hair line is marked； straight when the siphon is stationary； but curved when the siphon is pulled from side to side by the oscillations of the signal coil。
It is to the mouse…mill that me must look both for the electricity which is used to electrify the ink and for the motive power which drives the paper。 This unique and interesting little motor owes its somewhat epigrammatic title to the resemblance of the drum to one of those sparred wheels turned by white mice； and to the amusing fact of its capacity for performing work having been originally computed in terms of a 'mouse…power。' The mill is turned by a stream of electricity flowing from