78 



NATURE 



\May 2 2, 1879 



obtained to prove that a vast field of research wotild be 

 opened if a perfect induction balance could be found, 

 together with a means of correctly estimating the results 

 obtained. In experimenting with the microphone I had 

 ample occasion to appreciatej the exquisite sensitiveness 

 of the telephone to minute induced currents. This led 

 me to study the question of induction by aid of the tele- 

 phone and microphone. The results of those researches 

 have been already published.^ 



Continuing this line of inquiry, I thought I might again 

 attempt to investigate the molecular construction of 

 metals and alloys, and with this object I have obtained, 

 after numerous comparative failures, a perfect induction 

 balance which is not only exquisitely sensitive and exact, 

 but allows us to obtain direct comparative measures of 

 the force or disturbances produced by the introduction of 

 any metal or conductor. 



The instrument which I have the honour to present to 

 the Royal Society this evening, consists (i) of the new 

 induction-currents balance ; (2) microphone, with a clock 

 as a source of sound ; (3) electric sonometer, or absolute 

 sound measurer, a late invention of my own ; (4) a receiv- 

 ing telephone and three elements of Daniells' battery. 



In order to have a perfect induction-currents balance 

 suitable for physical research, all its coils, as well as the 

 size and amount of wire, should be equal. The primary 

 and secondary coils should be separate, and not super- 

 posed. The exterior diameter of the coils presented this 

 evening is 54 centims., having an interior vacant circular 

 space of 3i centims., the depth of this flat coil or spool is 

 7 millims. 



Upon this box-wood spool are wound ' 100 metres of 

 No. 32 silk-covered copper wire. I use four of such coils, 

 formed into two pairs, the secondary coil being fixed per- 

 manently, or by means of an adjustable slide, at a 

 distance of 5 millims. from its primary ; on the second 

 similar pair there is a fine micrometer screw, allowing 

 me to adjust the balance to the degree of perfection 

 required. 



These two pair of coils should be placed at 'a distance 

 not less than i metre from each other, so that no dis- 

 turbing cause should exist from their proximity. 



The two primary coils are joined in series to the 

 battery, the circuit also passing through the microphone. 

 In place of the telephone I have sometimes used a 

 magnetic pendulum, the swing or the arc described indi- 

 cating and measuring the forces.^ I am at present en- 

 gaged upon a very sensitive voltameter, which shall 

 indicate and measure the force of rapid induced currents. 

 The telephone, however, is well adapted as an indicator, 

 but not as a measurer of the forces brought into action. 

 For this reason I have joined to this instrument an instru- 

 ment to which I have given the name of electric sono- 

 meter. This consists of three coils similar to those 

 already described, two of which are placed horizontally 

 at a fixed distance of 40 centims. apart, and the communi- 

 cation with the battery is so arranged that there are 

 similar but opposing poles in each coil ; between these 

 there is a coil which can be moved on a marked sliding 

 scale divided into millimetres ; in a line with these two 

 opposing primary coils, the centre coil is the secondary 

 one, and connected by means of a circuit changing key 

 with the telephone in place of the induction-balance. If 

 this secondary coil is near either primary coil we hear 

 loud tones, due to its proximity. The same effect takes 

 place if the secondary coil is near the opposing coil, 

 except that the induced current is now in a contrary 

 direction, as a similar pole of the primary acts now on 

 the opposite side of the induction-coil ; the consequence 

 is that as we withdraw it from one coil approaching the 

 other, we must pass a line of absolute zero, where no 

 current whatever can be induced, owing to the absolute 



' Comptes Rendjts, December 30, i£ 

 Telegraph Engineers, M.irch 12, 1879. 



^ Telegraphic Journal, December 15, 1878. 



and January 20, 1879 ; Scciety 06 



equal forces acting equally on both sides of the induction- 

 coil. This point is in the exact centre between the two 

 coils, no matter how near or distant they may be. We 

 thus possess a sonometer having an absolute zero of 

 sound ; each degree that it is moved is accompanied by 

 its relative degree of increase ; and this measure may be 

 expressed in the degrees of the millimetres passed 

 through, or by the square of the distances in accordance 

 with the curve of electro-magnetic action. 



If we place in the coils of the induction-balance a piece 

 of metal, say copper, bismuth, or iron, we at once pro- 

 duce a disturbance of the balance, and it will give out 

 sounds more or less intense on the telephone according 

 to the mass, or if of similar sizes, according to the mole- 

 cular structure of the metal. The volume and intensity 

 of sound is invariably the same for a similar metal If 

 by means of the switching-key the telephone is instantly 

 transferred to the sonometer, and if its coil be at zero, we 

 should hear sounds when the key is up or in connection 

 with the induction-balance, and no sounds or silence 

 when the key is down or in connection with the sono- 

 meter. If the sonometer-coil was moved through several 

 degrees, or through more than the required amount, we 

 should find that the sounds increase when the key is 

 depressed ; but when the coil is moved to a degree where 

 there is absalute equality, if the key is up or down, then the 

 degree on scale should give the true value of the disturb- 

 ance produced in the induction-balance ; and this is so 

 exact that if we put, say, a silver coin whose value is 115°, 

 no other degree will produce equality. Once knowing, 

 therefore, the value of any metal or alloy, it is not neces- 

 sary to know in advance what the metal is, for if its 

 equality is ilj", it is silver coin ; if 52, iron ; if 40, lead ; 

 if 10, bismuth ; and as there is a very wide limit between 

 each metal, the reading of the value of each is very rapid, 

 a few seconds sufficing to give the exact sound-value of 

 any metal or alloy. 



During the course of these experiments with this in- 

 strument I noticed that my own hearing powers varied 

 very much with state of health, weather, &c., that differ- 

 ent individuals had wide differences of hearing, and that 

 nearly in all cases one ear was more sensitive than the 

 other ; thus whilst my degree of hearing was 10°, another 

 might be 60 in one ear and 15 in the other.' 



Dr. Richardson, F.R.S., who upon my invitation inves- 

 tigated this subject, became so impressed with the value 

 of the instrument as an absolute measurer of our hearing 

 powers, and its capabilities of throwing much light upon 

 its relation with health, that he has undertaken a series 

 of researches which will extend over some time, and 

 which I think, from some facts already gained, will be of 

 great value to the medical profession. These experi- 

 ments are now in his very able hands, and he will in due 

 time announce the results to the Royal Society. 



If an observer's hearing is limited to 10°, how can we 

 hear results below this line ? I should have stated that 

 when used to measure the hearing power, we determine 

 on a constant standard of force such as one element 

 Daniell, but if we increase the number of elements we in the 

 same ratio increase the inductive disturbance, and thus by 

 a large increase of force bring within our range results too 

 feeble to be heard without its aid, the sonometer con- 

 stantly, however, giving the same degree for equality as 

 the increased force is also used on this instrument. Thus 

 in our measurements we can entirely neglect the amount 

 of battery, as its comparative results remain a constant. 



As a rule three Daniell elements will be found quite 

 sufficient, and even this weak force is so exquisitely 

 sensitive that it will find out the smallest fraction of 

 difference in weight or structure of metals. Thus two 

 silver coins such as a shilling, both quite new, and both 

 apparently of the same weight, will be found to possess a 



' To this portion of my instrument when used as a measurer of our hear- 

 LDg powers, we have given the name o£ audiometer. 



