NATURE 



\jfune 29, 1882 



me that a few details, mainly constructive, might prove 

 of interest. It was suggested by Mr. Preece, in conse- 

 quence of a statement made by me in a paper on " Mea- 

 surement in the Medical Application of Electricity," read 

 before the Society of Telegraph Engineers. This state- 

 ment was to the effect that some difficulty still existed in 

 the trustworthy estimation of induction currents of 

 medium strength, such as are habitually used for physio- 

 logical and therapeutical purposes. The French Inter- 

 nationa! Co-nmission had only imperfectly remedied the 

 defhiencv by recommending the universal adoption of a 

 particular pattern of induction coil made by a single 

 German firm, and arbitrarily graduated to a "sledge" 

 apparatus. Mr. Preece thought that a dynamometer, 

 which may be regarded as a galvanometer of which the 

 moving magnet is replaced by a suspended coil introduced 

 into the circuit, would answer the purpose ; since the de- 

 flection of the coil is in one uniform direction, although 

 the currents traversing the circuit are alternate. This 

 very practical hint seemed to offer a prospect of obtaining 

 accuracy in a department of science in which it is much 

 needed. But on examining existing dynamometers 1 

 found only Weber's original instrument, which, in spite 

 of its immense value, is fitted only for a well-appointed 

 laboratory, and another, made by Messrs. Siemens for 

 the measurement of very intense electric light currents, 

 which erred on the opposite side of deficient delicacy. 

 The dynamometer of Messrs. Siemens, shown at the 

 F'rench International Exhibition, by means of which the 

 alternating currents of telephones were demonstrated, 

 was probably in the same category, though neither I, nor 

 the president of the Physical Society could obtain any 

 e\act details of its internal arrangements. 



An electrodynamic balance, described in the Annalen 

 tier Physik in 1SS1 by Helmholtz, comes somewhat closer 

 to the requirements of the case, but this, like that of 

 Weber, is a delicate apparatus, difficult of transportation. 

 It might, however, prove excellent as a means of cali- 

 brating a less perfect and absolute, but more handy 

 instrument, such as that I was in search of. 



Another form of dynamometer had been incidentally 

 named to me by Mr. Ayrton, of his invention, in which 

 the moving coil is replaced by a piece of soft iron which 

 becomes magnetic during the passage of the current. Of 

 this also further details were wanting. 



I therefore attempted to make one for myself by the 

 usual method of suspending a coil of wire from two silk 

 fibres within a fixed coil, bringing its two ends to mercury 

 contacts at the lower part, and joining all up in one 

 circuit. Two defects at once appeared. I. The coil of 

 copper wire was fur too heavy to move with the small 

 currents at my disposal ; and when it did swing, it con- 

 tinued to oscillate slowly for an unlimited time, giving no 

 satisfactory reading. 2. The mercury contacts caused so 

 much friction as absolutely to stop all motion whatever. 



It was therefore obvious that (1) a light coil, and (2) a 

 sensitive bifilar suspension were needed. Both of these 

 must have a fairly high electrical conductivity. The 

 second of these desiderata may be dismissed first. I 

 found at the gold lace shops bobbins of silver-gilt wire, in 

 which the gold is drawn over the silver in manufacture ; 

 not merely plated on. These two combined have a dia- 

 meter of ^Jj ff of an inch ; which is exactly that of the finest 

 platinum wire commercially made. But whereas the 

 resistance of 1 metre of the latter is 62-2 ohms, that of the 

 former is only o/S ohms. An induction shock from 

 Dubois-Reymond's apparatus passed through a metre of 

 this wire has such strength, that I do not wish to try it 

 again, nor should I venture to administer it to an 

 invalid. 



It occurred to me that (1) the light coil might be 

 obtained by using fine aluminium wire covered with silk. 

 Messrs. Johnson and Matthey, with their usual cour.'esy, 

 drew this forme specially, to a diameter of i-iooth of an 



inch, or even less, 1 and Mr. Rickards, of Derby, com- 

 pleted the operation. 



By winding this on a mandril, tying the ring thus 

 obtained with silk threads, and immersing in photo- 

 graphic amber varnish, which I find much less dense, 

 and as good an insulator as Shellac, I obtain a coil 

 composed entirely of metal and silk, which is at once 

 rigid, light, and conductive. One of these, of 1 '25 inches 

 internal diamater, not of very fine wire, contains forty-two 

 turns of wire in five layers, its length thus being over four 

 yards. It weighs 6'25 grammes, less than 100 grains, its 

 resistance is about half an ohm. 



On suspending this light coil from two threads of the 

 silver-gold wire named above, I found its deflections con- 

 siderable, and easily measured, even with moderate cur- 

 rents. It could easily be made "dead-beat." The bifilar 

 couple was varied by giving the suspending points a 

 sliding motion to and from each other. By also fashion- 

 ing the suspensions in the form of light vertical springs, 

 the two threads were kept at an approximately equal 

 tension. 



Aluminium appears to offer great advantages for em- 

 ployment in such functions as these. It is said in 

 Watt's "Dictionary," that "the electric conducting power 

 of aluminium is eight times as great as that of iron, and 

 about equal to that of silver," where probably the com- 

 parison is intended to be made with equal weights, and 

 not volumes. But even if it were lower, it would be 

 abundantly sufficient for the purpose named, as the cur- 

 rents are of high tension, and as the resistance need 

 not be materially less than that of the suspending wires 

 given above. Its specific heat is very great, so that 

 moderate changes of temperature affect it but little. This 

 property might render it valuable for the fabrication of 

 resistance coils. 



It was stated at the meeting that this metal had been 

 tried by Messrs. Siemens, but given up in consequence 

 of the failure of connection in the ends of the aluminium 

 wire. This difficulty I have not found, probably in con- 

 sequence of the high tension, and also from the fact that 

 the contacts are between gold and aluminium, both stable 

 substances. In any case the difficulty could be overcome 

 by making a gripping contact with a light clamp, such as 

 is already used in watchwork. Nor can Messrs. Siemens' 

 unsuccessful attempt for other purposes be, I think, con- 

 sidered as a distinct anticipation of this. The mechani- 

 cal advantage of such a light coil in diminishing moment 

 of inertia, and in reducing the force of the bifilar couple, 

 can hardly be denied on theoretical grounds, and is, 

 indeed, borne out by experiment. W. H. STONE 



MATHEMATICS AT THE JOHNS HOPKINS 

 UNIVERSITY 



FROM time to time we receive copies of the University 

 Circular. From two now before us, we make a few 

 extracts, which will serve to show what this young but 

 promising University has done (or attempted to do) in 

 the session 1S81-S2. ' The students have been thirty-two 

 in number ; of these, twenty followed advanced and 

 University courses, and twelve pursued collegiate courses. 



Supreme over the department presides Prof. Sylvester, 

 F.R.S., who, besides editing the American Journal of 

 Mathematics and reading papers at the Mathematical 

 Seminary (similar in its character to OCT own London 

 Mathematical Society), 'has delivered two courses of 

 lectures— one on the Theory of Numbers (and in especial 

 on an extension of Tchebycheff 's theory concerning Prime 

 Numbers), the other on a new theory of universal multiple 

 algebra. 



This session on the invitation of the Trustees, Prof. 

 Cayley was called in as amicus curia, and arrived at 



1 The finest wire has not yet been measured in the microscope ; it passes 

 through the smallest hole of the B.W.G.. No. 80. 



