April 15, 1875] 



NA TURE 



471 



When the poles of two permanent-magnets are opposed 

 to each other, the similar poles will exert a repellant, and 

 the dissimilar poles an attractivt force. This principle is 

 constant, whether the magnets are electro-magnets or 

 permanent-magnets. 



In a permanent-magnet, as is well understood, the 

 magnetic force culminates at the two opposite extremities 

 of the bar, and for the purposes of telegraphy may be 

 considered as equivalent to the force emanating from the 

 two poles of a voltaic series, but more lasting ; there is no 

 battery to be renewed, the excitation of the current is 

 mechanical, and not chemical. 



When a piece of soft iron is placed close to the poles 

 of a permanent-magnet (Fig. 15), it will become a magnet 

 by induction, and the polarity of the ends will be dissimilar 

 in their nature to those of the permanent-magnet. 



Fig. 16. — Magnetisation of pieces of soft iron by the influ 

 (induction). 



; of magnetic 



When the pole of a permanent-magnet is placed 

 within a hollow coil or helix of insulated wire freely sus- 

 pended so as to oscillate on an axis, and a current of elec- 

 tricity is passed through the helix, it will be oscillated or 

 rotated towards the right or left over the poles of the 

 magnet according to the direction of the current. 



In a similar way, when a permanent magnetic bar is 

 freely suspended within a hollow coil or helix of wire, the 

 magnetic bar will oscillate to the right or left, according to 

 the direction in which the current flov/s through the helix. 



These are the principal fundamental laws which, com- 

 bined together in various mechanical details, constitute 

 every form of telegraphic apparatus known; and it is upon 

 the accurate balance of the resistances, and delicacy of the 

 mechanical parts, that the excellence of the instrument 

 for practical purposes depends. It will now be pointed 

 out how these well-known principles have been combined 

 to produce the beautiful machines at present employed 

 upon submarine circuits of extended length, and by which, 

 with feeble currents, signals are automaticaJly recorded at 

 the distant station. 



Commencing with non-recording instruments, the 

 mirror galvanometer is at once the most useful and im- 

 portant in its general applications to submarine telegraphy. 

 The electrical combinations of principles which constitute 

 this instrument existed almost in the same arrangement 

 in the earliest days of telegraphic research. At that early 

 period the apparatus in its elementary conditions was 

 almost identical with the modem instrument, the bar- 

 magnet freely suspended in the centre of a hollow coil of 

 insulated wire, and the focal distance at which to observe 

 the angular motion of the suspended needle to the right 



or left. In this crude arrangement there existed the germ 

 of the instrument now in use, the accurate balance of 

 resistances, and delicate adjustments of the mechanical 

 parts, producing the difference between an historical in- 

 vention and an every-day practical mechanical application. 

 The construction of the reflecting galvanometer is exceed- 

 ingly simple, the delicacy of the instrument being the 

 result of the lightness of the moving parts. 



Two hollow coils of fine wire (Fig. 17), united to form a 

 continuous circuit, are placed one above the other, and 

 the coils are so constructed as to admit of a very deli- 

 cate axis being inserted through them free to rotate and 

 capable of accurate adjustment, so that the centre of 

 rotation may be in a line with the centre of the inner 

 ring of the coils. A minute silvered mirror reflector is 

 attached to the axis concentric with the hollow centre 

 of the upper coil. Two extremely light bar-magnets, 

 about three-eighths of an inch in length, are attached to 

 the axis in the centre of each coil, one of the magnets 

 being therefore at the back of the mirror. The polarity 

 of these bar-magnets is reversed, producing an astatic 

 combination. The whole arrangement of axis, mirror, 

 and bar-magnets is suspended by a cocoon fibre, adjust- 

 ments being obtained to ensure freedom oi rotation by a 

 micrometer screw and levelling screws. The mirror 

 is brought into the field and its motion otherwise co:i- 

 troUed by means of a permanent-magnet sliding upou 

 the rod, the elevation or depression of which acting by 

 induction upon the suspended bar-magnets gives more 

 or less sensitiveness to the mation of the mirror when .1 

 current of electricity traverses the coils. It must be 

 obvious that the eye is quite incapable of detecting 

 with accuracy the minute angular motions of the mirror, 

 and that some means must be employed to magnify and 

 increase this angular motion of the magnetic bar. For 

 this purpose a beam of light is employed which, falling 

 on the mirror, is reflected bick again upon a long hori- 

 zontal scale placed some six feet off. The angle of inci- 

 dence of the beam of light being equal to the angle of 

 reflection, the oscillation of the mirror thus magnified to 

 the eye, to right or left, is read oft' from a zero on the 

 scale. The beam of light is passed through an adjusting 

 slit immediately beneath the scale, and the mirror is 

 brought to the zero of the scale by the magnetic adjust- 

 ment before mentioned. Thus the slightest angular motion 

 of the mirror, inappreciable to the eye, is, according to 

 the focal length of the ray of light, increased to such an 

 extent as to indicate the presence of the most feeble cur- 

 rents with an almost inappreciable movement of the 

 mirror. 



The scientific world is indebted to Prof Sir William 

 Thomson for this exceedingly beautiful adaptation and 

 combination of existing laws, parts, and principles ; the 

 skilful balance of which has resulted in an apparatus 

 now almost exclusively used for the testing of the electric 

 condition of submarine cables. It is obvious that with 

 this reflecting galvanometer no automatic register of the 

 signals received can be obtained ; recourse is therefore 

 had to a Morse key, by means of which the recipient of the 

 signal at once records the deflection of the light spot on 

 the scale to the right or left in the symbolic Morse code 

 of the dot and dash. The mirror galvanometer, in fact, 

 occupies relatively the same position in electrical mecha- 

 nics as the violin does in musical acoustics. In the violin, 

 by sliding the finger up the string and thus shortening the 

 length of the vibrating string, the musical pitch or tone 

 produced from the string, as the bow is drawn across it, 

 continues to ascend in the musical scale without break ; 

 each note of the entire diatonic scale capable of being 

 produced on that string, sliding or melting into the next, 

 the pitch of the note being the index or record of the 

 length of the string and numerical value of its vibrations. 



In a similar manner the great peculi.irity of the 

 mirror or reflecting gal\'anonietcr is, that it contiauously 



