520 



NATURE 



[lifarch 31, 1 88 1 



iaKtrument for producing the new induction-current ; (2) sono- 

 meter or balancing coils; (3) rheotome and battery; (4) 

 telephone. 



The essential portion of this new balance is that wherein 

 a coil is so arranged that a wire of iron or copper can 

 pass freely through and forming it<; axis, the iron or copper 

 wire rests upon two supports 20 centims. apart ; at one of these 

 the wire is firmly clamped by two binding screws ; the opposite 

 end of the wire turns freely on its support, the «ire bein^; 22 

 centims. long, having 2 centims. projection beyond its support, 

 in order to fasten upon it a key or arm which shall serve as a 

 pointer upon a circle givm'^ the degrees of torsion which the 

 wire receives from turning this pointer. A binding screw allows 

 us to fasten the pointer at any degree, and thus preserve the 

 required stress the time required. 



The exterior diameter of the coil is 54 centim'., having an 

 interior vacant circular space of 3 J centims., its width is 2 

 centims. ; upon this is wound 200 metres uf No. 32 silk-covered 

 copper wire. This coil is fastened to a small board so arranged 

 that it can be turned through any desired angle in relation to the 

 iron wire which passes through its centre, and it can also be 

 moved to any portion of the 20 centims. of wire, in order that 

 different portions of the same wire may be tested for a similar 

 stress. 



The whole of this instrument, as far as possible, should be 

 constructed of wood, in order to avoid all disturbing inductive 

 influences of the coil. 



The iron wire at its fixed end is joined or makes contact with 

 a copper wire, which returns to the front part of the dial under 

 its board and parallel to its coil, thus forming a 1 ^op, the free 

 end of the iron wire is joined to one pole of the battery, the 

 copper wire under the board is joined to the rheotome and 

 thence to the battery. 



The coil is joined to the telephone; but, as in every instance 

 we can either pass the battery through the wire, listening to its 

 inductive effects upon the wire, or the reverse of this, I prefer, 

 generally, in order to have no voltaic current passing through 

 the wire, to join the iron wire and its loop direct to the tele- 

 phone, pasNing the voltaic current through the coil. 



In order to balance, measure, and know the direction 

 of the new induction currents by means of a switching key, 

 the sonometer (Fmc. Roy. Soc, vol. xxix. p. 65) I described 

 to the Royal Society is brought into the circuit. The two 

 exterior coil.s of the sonometer are then in the circuit of tlie 

 oattery, and of the coil upon the board containing the iron wire 

 or stress bridge. The interior or movable coil of the sonometer 

 is then in the circuit of the iron wire and telephone. Instead of 

 the sonometer constructed as described in my pa(_er to the Royal 

 Society, I prefer to use one formed upon a principle I described 

 m Comptes rendus, December 30, 1878. This consists of two 

 coils only, one of which is smaller and turns freely in the centre 

 of the outside coil. The exterior coil being stationary, the centre 

 coil turns upon an axle by means of a long (20 centims.) ami or 

 pointer, the point of which moves over a graduated arc or circle. 

 Whenever the axis of the interior coil is perpendicular to the 

 exterior coil no induction takes place, and « e have a perfect 

 zero ; by turning the interior coil through any degree we have a 

 current proportional to this angle, and in the direction in whicli 

 it is turned. As this instrument obeys all the well-known laws 

 for galvanometers, the readings and evaluations are easy and 

 rapid. 



If the coil upon the stress bridge is perpendicular to the iron 

 wire, and if the sonometer coil is at zero, no currents or sounds 

 in the telephone will be perceived, but the sliijhtest current in 

 the iron wire produced by torsion will at once be heard ; and by 

 moving the sonometer coil in a direction corresponding to the 

 current, a new zero will be obtained, which will not only balance 

 the force of the new current, but indicate its value. A perfect 

 zero however will not be obtained with the powerful currents 

 obtained by the torsion of 2 millims. diameter iron wire ; we then 

 require special arrangements of the sonometer, which are too 

 complicated to describe here. 



The rheotome is a clockwork having a rapid revolving wheel 

 which gives interruptions of currents in fixed cadences in order 

 to have equal intervals of sound .and silence. I employ four 

 bichromate cells or eight Daniell's elements, and they are joined 

 through this rheotome to the coil on the stress bridge, as I have 

 already described. 



The magnetic properties of iron, steel, nickel, and cob.-iIt 

 have been so searchingly investigated by ancient as well as by 



modern scientific authors, that there seems little left to be known 

 as regards its molar magnetism. I use the word molar here 

 simply to distinguish or separate the idea of a magnetic bar of 

 iron or steel magnetised longitudinally or transversely from the 

 polarised molecules which are supposed to produce its external 

 magnetic effects. 



Molar magnetism, whilst having the power of inducing an 

 I electric current in an adjacent wire, provided that either has 

 [ motion or a change in its magnetic force, as shown by Faraday 

 ' in 1832, has no power of inducing an electric current upon 

 itself or its own molar constituent, either by motion or change 

 of its magnetic moment. Molecular magnetism ('he results of 

 which I believe I have been the first to obtain) has no, or a very 

 feeble, power of inducing either magnetism or an electric current 

 in an adjacent wire, but it possesses the remarkable power of 

 strongly reacting upon its own molar wire, inducing (compara- 

 tively with its length) powerful electric currents, in a circuit of 

 which this forms a part. 



VVe may have also both cases existing in the same wire ; this 

 occurs when the wire is under the influence of stress, either 

 external or internal ; it would have been most difficult to separate 

 these two, as it was in my experiments w ith the induction balance 

 without the aid of my new method. 



Ampere's theory supposes a molecular magnetism or polarity, 

 .and that molar magnetism would be produced when the molecu- 

 lar magnetism became symmetrical ; and his theory I believe is 

 fully capable of explaining the effects I have obtained, if we 

 admit that we can rotate the paths of the polarised molecules by 

 an elastic torsion. 



Matteucci made use of an inducing and secondary coil in the 

 year 1847 (Comft. rend. t. xxiv. p. 301, 1847), by means of 

 which he observed that mechanical strains increased or decreased 

 the magnetism of a bar inside this coil. 



Wertheim published in the Comptes rendus, 1852 (Compt. rend. 

 t. XXV. p. 702, 1852), some results similar to Matteucci ; but in 

 the Annates de Chimie et de Physique, 1S57 (Ann. de CJiim. et de 

 Phys. (3) t. I. p. 385, 3857), he published a long series of most 

 remarkable experiments, in which he clearly proves the influ' 

 ence of torsion upon the increment or decrement of a magnetical 

 wire. 



Vilari showed {Poggendorff' s_ Annalen, 1868) increa.se or 

 diminution of magnetism by longitudinal pull according as 

 the magnetising force is less or greater than a certain critical 

 value. 



Wiedermann (Wiedermann's " Galvanismus," p. 447), in his 

 remarkable work, "Galvanismus," says that an iron wire 

 through which an electric current is flowing becomes magnetised 

 by twisting the wire. This I have repeated, but found the effects 

 very weak, no doubt due to the weak battery I use, viz. four 

 quart bichromate cells. 



.Sir W. Thomson shows clearly in his remarkable contribution 

 to the Pliit. Trans. Roy. Soc, entitled " Effects of Stress on 

 I the Magnetisation of Iron, Nickel, and Cobalt " (Pint. Trans. 

 May 6, 1 878), the critical value of the magnetisation of these 

 metals under varying stress, and also explains the longitudinal 

 magnetism produced by Wiedermann as due to the outside molar 

 tuist of the wire, making the current pass as in a s|iiral round a 

 fixed centre. .Sir William Thomson also shows clearly the effects 

 of longitudinal as well as transversal strains, both as regards its 

 molar magnetism and its electric conductivity. 



My own researches convince me that \ye have in molecular 

 magnetism a distinct and separate form of magnetism from that 

 when we develop, or render evident, longitudinal or transversal 

 mngnetism, which I have before defined as molar. 



Molecular magnetism is developed by any slight strain or twist 

 other than longitudinal, and it is only developed by an elastic 

 twist ; for however much we may twist a wire, provided that its 

 fibres are not separated, we shall only have the result due to the 

 reaction of its remaining elasticity. 



If we place an iron wire, say 20 centims. long, I millim. dia- 

 meter, in the axis of the coil of the electro-magnetic balance, 

 and if this wire is joined, as described, to the telephone, we find 

 that on passing an electric current through the inducing coil no 

 current is perceptible upon the iron h ire ; but if we give a very 

 slight twist to this wire at its free end — one-eighteenth of a turn, 

 or 20°— we at once hear, clear and comparatively loud, the cur- 

 rents passing the coil ; and although we only gave a slight elastic 

 twist of 20° of a whole turn, and this spread over 20 centims. in 

 length, making .an extremely slight molar spiral ; yet the effects 

 are more powerful than if, using a wire free from stress,, we 



