March 31, 1881J 



NA rURE 



521 



turned the whole coil 40°. The current obtained when we turn 

 the coil, as juat meiitioned, is secondary, and with the coil at 

 any angle any current produced by its action, either on a copper, 

 silver, iron, or steel wire ; in fact it is simply Faraday's discovery, 

 but the current from an elastic twist is no longer secondary under 

 the same conditions, but tertiary, as I shall demonstrate later on. 

 'ihe current passing through the coil cannot induce a current 

 upon a wire perpendicular to itself, but the molecules of the 

 outside of the wire, being under a greater elastic stress than the 

 Tvire itself, they are no longer perpendicular to the centre of the 

 wire, and consequently they react upon this wire as separate 

 magnets would upon an adjacent wire. It might here be readily 

 suppo>ed that a wire having several twists, so a fixed molar twist 

 of a given amount would produce similar eflect^. It however 

 does not, for in most cases the current obtained from the molar 

 twists are in a contrary direction to that of the elastic torsion. 

 Thus, if 1 place an iron wire under a right-handed elastic twist 

 of 20° I find a positive current of 50° sonometer ; but if I 

 continue this twist so that the index makes one or several entire 

 revolutions, thus giving a permanent molar twist of several 

 turns, I find upon leaving the index free from any elastic 

 torsion, that I have a permanent current of 10°, but it is no 

 longer positive but negative, requiring that we should give an 

 elastic torsion in the previous direction, in order to produce a 

 positive current. Here a permanent elastic tor-ion of the mole 

 cules is set up in the contrary direction to its molar twist, and we 

 have a negative current, overpowering any positive current which 

 should have been due to the twisted wire. 



The following table shows the influence of a permanent twist, 

 and that the current obtained when the w ire was freed from its 

 elastic torsion was in opposition to that which should have been 

 produced by the permanent twist. Thus a well-softened iron 

 wire I miUim. in diameter, givi g 60" positive current for a 

 right handed elastic torsion of 20", gave after 1^*80 permanent 

 torsion a negative current of 10°. 



1 complete permanent torsion (right-handed) negative ... 10 



2 „ „ „ ... 15 



3 .. .. ,. 15 



4 .. ., ., 16 



7 .. .. .. ■■ 5 



8 „ „ „ ... 4 



9 .. .. .. •-■ 3 



10 „ „ ,, .. 3 



At this point the fibres of a soft wire commence to separate, 

 and we have no longer a complete single wire, but a helix of 

 separate wires upon a central structure. 



If now, instead of passing the current through the coil, I pass 

 it through the wire, and place the telephone upon the coil 

 circuit, I find that I obtain equally as strong tertiary currents 

 upon the coil as in the previous case, although in the first case 

 there was produced longitudinal electro-magnetism i'l the per- 

 pendicular wire by the action of the coil, but in the latter case 

 none or the most feeble electro-magnetism was produced, yet in 

 these two distinct cases we have a powerful current produced 

 not only upon its own wire, but upon the coil, thus proving that 

 the effects are equally produced both on the wire and coil. 



If we desire, however, in these reversible effects to produce in 

 both cases the same electromotive force, we must remember that 

 the tertiary current when reacting upon its own short wire 

 produces a current of great quantity, the coil one of comparative 

 higher intensity. We can, however, easily convert the great 

 quantity of the wire into one of higher tension by passing it 

 through the primary of a small induction coil whose resistance 

 is not greater than one ohm. We can join our telephone, 

 which may be then one of a high resistance, to the secondary of 

 this induction coil, and by this means, and without changing the 

 resistance of the telephone, receive the same amount of force, 

 either from the iron wire or the coil. 



Finding that iron, steel, and all magnetic metals produce a 

 current by a slight twist, if now we replace this wire by one of 

 copper or non-magnetic metals we have no current whatever by 

 an elastic twist, and no effects, except when the wire itself is 

 twisted spirally in helix ; and whatever current we may obtain 

 from copper, &c., no matter if from its being in spiral or from 

 not being perpendicular to the axis of the coils, the currents 

 .obtained will be invariably secondary and not tertiary. If we 

 replace the copper by an iron wire, and give it a certain fixed 

 torsion, not passing its limit of elasticity, we find that no in- 



crease or decrease takes place by long action or time of being 

 under strain. Thu- a wire which gave a sonometric force of 

 50° at the first observation remained perfectly constant for seve- 

 ral days until it was again brought to zero by taking off the 

 strain it had received. Thus we may consider that as long as 

 the wire preserves its elasticity, exactly in the same ratio will it 

 preserve the molecular character of its magnetism. 



It is not necessary to use a wire to produce the^e effects ; still 

 more powerful currents are generated in bars, ribbons, or sheets 

 of iron ; thus no matter what external form it may possess, it 

 still produces all the effects I have described. 



It requires a great many permanent twists in a wire to be 

 able to see any effect from the-e twists, but if we give to a wire, 

 I millim. diameter, forty whole turns (or until its fibres bee jine 

 separated) we find some new effects ; we find a small current of 

 10° in the same direction as its molar twist, and on giving a 

 slight twist (20") the sonometric value of the sound obtained is 

 80° instead of 50°, the real value of a similar untwiste i wire ; 

 but it- explanation will be found by twisting the wire in a c^ni- 

 trary direction to its molar twist. We can now approach the 

 zero but never produce a current in the contrary direction, owing 

 to the f;ict that by the spiral direction, due to the fibrous mcilar 

 turns, the neutral position of its molecule is no longer parallel 

 with its wire, but parallel with its molar twist, consequently an 

 elastic strain in the latter case can only bring the molecules 

 parallel with its wire, producing no current, and in the first case 

 the angle at which the reaction takes place is greater than belorc, 

 consequently the increased value of its current. 



The measurements of electric force mentioned in this paper are 

 all sonometric on an arbitrary scale. Their absolute value has 

 not yet been obtained, as we do not, at our present stage, require 

 any except comparative measures.' Thus, if each wire is of i 

 milHni. diameter and 20 centims. long, all render the same stress 

 in the axis of its coil. I find that the following are the sonometric 

 degrees of value ; — 



60 Tertiary current. 

 45 



Soft iron ... 



Hard drawn iron 



Soft .steel 



Hard tempered steel .... 



Copper, silver, &c -o ,, ,, 



Copper helix, I centim. diameter, 



20 turns in 20 centims 45 Secondary currents. 



Iron, spiral, ditto 45 ,, ,, 



Steel 45 ,, ,, 



The tertiary current increases with the diameter of the wire, 

 the ratio of which has not yet been determined ; thus an ordinary 

 hard iron wire of I millim. diameter giving 50", one of 2 millims. 

 diameter gave 100° ; and the maximum of force obtained by any 

 degree of torsion is at or near its limit of elasticity, as if in the 

 same time we also pass this point, producing a permanent twi-t, 

 the current decreases, as I have already shown in the case of a 

 permanent twist. Thus, the critical point of I millim. hard iron 

 wire was 20° of torsion, but in hard steel it was 45°. 



Longitudinal strains do not produce any current whatever, but 

 a very slight tw ist to a wire, under a longitudinal strain, pro- 

 duce ; its maximum effects : thus, 20° of torsion being the critical 

 point of iron wire, the same wire, under longitudinal strnm, 

 required but from 10° to 15°. It is very difficult however to 

 produce a perfect longitudinal strain atone. I have thercf.re 

 only been able to try the effect of longitudinal strain on fine 

 wires, not larger than I millim in diameter, but as in all cases no 

 effect whatever was produced by longitudinal strain alone, I 

 believe none will be found if absolutely free from torsion. The 

 molecules in a longitudinal strain are equally under an ela-nc 

 strain as in torsion, but the path of their motion is now parallel 

 with its wire, or the zero of electric inductive effect, but the 

 compound strain composed of longitudinal and transverse, rea t 

 upon each other, producing the increased effect due to the 

 compound strain. 



The sonometer is not only useful for showing the direction of 

 the current and measuring it by the zero method, but it as' 

 shows at once if the current measured is secondary or tenia y 

 If the current is secondary its period of action coincides » ith 

 that of the sonometer, and a perfect balance, or zero of sou d. 

 is at once obtained, and its value in sonometric degrees given, 

 but if the current is tertiary, no zero is possible, and if the valu- 

 of the tertiary is 60°, we find 60° the nearest approach to zer 



' 50° sonometer has the same electromotive force as ©"'lo of a Daniell 

 battery. 



