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XXVII. The Clark Cell when Producing a Current.

To the Editors of the Philosophical Magazine. GENTLEMEN, IN the Philosophical Magazine for September 1894 is a

Producing a Current.” This paper appears to call for one or two remarks from me. Mr. Skinner says (p. 272)“Let a cell of

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. electromotive force E and internal resistance R have its poles joined by a wire of resistance r ; then, providing R and r are constant, and there is no polarization, the potential-difference

TE between the poles will be If, however, there is po

R+7 larization, then this potential-difference will be where

R+p? e is the value of the electromotive force required to produce the observed current.”

The value of — (E-e) is the “electromotive force of polarization.” This quantity is studied by Mr. Skinner, and its evaluation of course involves measurements of Rand ?'. With regard to r', which appears to have been taken from a a box for values down to 200 legal obms, and for 147 legal ohms from a german-silver coil in a bath of paraffin oil, we are not given very much information. I would ask, however, whether in the light of Mr. Griffiths' experience (Phil. Trans. A. 1893, p. 401), it is safe to assume that the resistance of a coil

. mounted as described, and carrying a current, can be sufficiently ascertained by tests made with only a testing-current. No one ought to be more alive to this than Mr. Skinner, so that I will assume his value of r to have been sufficiently known. With regard to “R,” however, the case is different. Its value was measured by Professor Macgregor's method (Trans. Roy. Soc. Canada, iii. 1882, p. 22, referred to as " by means of the commutator used by Mr. T. C. Fitzpatrick”), in which alternating intermittent currents are fed into a Wheatstone bridge, and commutated for the benefit of a galvanometer. The measurements were not made while the cell under examination was yielding a current for polarization observations : consequently the value assigned to

-(E-e) rests on the assumption that the resistance of the cell, while yielding a considerable current (01 ampere) for polarization experiments, was the same as the resistance it exhibited while under test by alternating and intermittent currents. This assumption I consider to be wholly illegitimate, and to vitiate every result given by Mr. Skinner for

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the value of -(E-e). In the present state of our informa

tion we are not entitled to make any assumption whatever on this question.

On page 277 Mr. Skinner gives a curve showing the behaviour of a Clark cell when continuously short-circuited so as to yield about :01 ampere. The drop of P.D. is shown in this curve to increase continuously. From this it appears to be inferred (conclusion “”, page 278) that “the electromotive force of polarization slowly increases when the current is maintained;” and further on (page 279) Mr. Skinner says: " In the experiments of Threltall the sign of the term depending on time was found to be negative. In some of my earlier experiments it appeared to be negative, but this was traced to irregularity in the working of the compensator; and the effect has always been positive since the Clark cells have been used in the place of the Leclanché.”

Now Mr. Pollock and I specially guarded ourselves in the paper referred to froin any statement whatever as to the

electromotive force of polarization,” having the fear of Lord Rayleigh and Professor Ayrton very properly before our eyes. Secondly, we fortunately have all our notes intact, and find that we kept a much better watch on the compensator than Mr. Skinner appears to have done ; and there is no doubt whatever that, in the case we examined, the P.D. drop diminished with time, and this was to be explained neither by change of compensator nor by heating of shortcircuiting resistance. We cannot ask the Editors to afford space for the voluminous numerical evidence to exhibit this rather unimportant fact; but we satisfied ourselves on this point in 1888, and since Mr. Skinner's paper appeared have gone into the matter again and find it to be as we stated in our paper:

As a matter of fact our cells were not made up quite in the same way as Mr. Skinner's, and the zincs were wrapped in parchment-paper, the resistance of which may, not improbably, have varied considerably. In instituting his comparison Mr. Skinner apparently overlooked this point, and this leads me to think that he did not refer to our paper very carefully, though probably as carefully as it deserved. We are the more inclined to this view from the statement appearing in $8,“ Conclusion," page 278, that “From the magnitudes of the quantities found in these experiments, it follows that small currents of approximately known value can be obtained by the use of large Clark cells of small internal resistance, which may be neglected in comparison with the large external resistance." Seeing that the primary object of our work was to discover whether such a proposition is true or not, that the greater part of our paper is taken up by a discussion as to the limits within which this proposition is true, that our results (much more numerous than Mr. Skinner's) are illustrated by two sheets of curves (plates xii. & xiii.), and, finally, that a method of determining high specific resistances based on this property of Clark cells hås been in use almost daily in our laboratory since the paper was written in 1888, and was described by us (Phil. Mag. 1889, vol. xxviii. p. 470)—considering all this, I say that Mr. Skinner's unconditioned statement of his conclusion appears to me to be somewhat inadequate.

There is yet another point. On page 273, and referring to the measurement of the resistance of his cells, Mr. Skinner says :

:-“However, following Mr. Fitzpatrick's method, tests were specially made for polarization by varying the ratio of the arms and by varying the speed of the commutator.” far as varying the speed of the commutator goes, this method was described to all intents and purposes by Kohlrausch in Pogg. Ann., Jubelband, p. 290, in Wiedemann's Electricität, vol. i. p. 476, and, I think, in Professor Chrystal's article in the Encyclopædia Britannica. It is therefore a mistake to ascribe it to Mr. Fitzpatrick.

R. THRELFALL. Sydney, December 30, 1894.

XXVIII. Value of the Magnetic Permeability for Rapid

Electrical Oscillations. To the Editors of the Philosophical Magazine. GENTLEMEN, THE NHERE have been various estimations of the above quan

tity, but the results resting upon experimental data are few. A late paper by Ignaz Klemenčič in Wied. Ann. der Physik und Chemie, No. 12, 1894, contains the following values of the permeability in case of oscillations of 1,000,000 per second:Soft iron

p=118, Steel

u=106 to 115.

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In a paper in the Philosophical Magazine for November 1894, on "Wave-Lengths of Electricity on Iron Wires," I

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obtained as a bye-result the average approximate value

u=385.

A recalculation of this value, however, shows an arithmetical error, which multiplies the value by 4, so that the true approximate value yielded by the data is u=96, ranging between the values 107 and 83:5, the extremes of the three values. These are lower than the results given by Klemenčič, but in my experiments the rate of oscillation was much higher.

In this connexion it is interesting to note that Mr. Oliver Heaviside, in his ' Electrical Papers,' vol. i. p. 361, “Waves of Magnetic Force," remarks that u is eminently variable, but that a fair average value is u=100. This assumption seems confirmed both from Klemenčič's results and my own.

I am, Gentlemen,

Yours respectfully,

CHARLES E. St. John. Berlin, Jan. 10, 189.5.

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XXIX. On the Liquefaction of Gases.

By Prof. J. DEWAR I

N the Philosophical Magazine for last month Professor relations to his alleged discoveries. It is usually assumed that if a scientific man has a grievance on some question of priority, he speaks out boldly at or about the time when his discovery is being appropriated. It seems, however, that Professor Olszewski prefers to nurse his complaints for four years and then to bring them out simultaneously in two English scientific journals. The result, I am afraid, will be grievously disappointing. Personally I am delighted to see an English edition of Professor Olszewski's papers. There is, however, one serious omission which I trust the Editors of the Philosophical Magazine may remove before long. We want in this country a reprint of the splendid papers of the late Professor Wroblewski. Until this is done it will be impossible for the scientific public to decide on many of Professor Olszewski's claims for priority. Now let me turn to the Professor's treatment of myself. The following are the deliberate statements made by him in his paper “ On the Liquefaction of Gases,” Philosophical Magazine, February 1895 :

* Communicated by the Author.

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“But in his last experiments and lectures respecting the liquefaction of considerable quantities of oxygen and air and their employment as cooling agents, Professor Dewar has thought fit not to make any mention of my labours in the same field, which had been published several years before Professor Dewar went over them again. In the number for June 1890 of the Bulletin International de l'Académie de Cracovie, I have described an apparatus serving to liquefy a greater quantity of oxygen or air in a steel cylinder, from which it can be poured out into an open glass vessel, and used as a frigorific agent. It is entitled “ K. Olszewski, Transvasement de l'oxygène liquide ;' and a brief report on the subject is contained in the Beiblätter of Wiedemann, vol xv. p. 29, under the title K. Olszewski. Ueber das Giessen des flüssigen Sauerstoffs.' That my labours should have thus been passed over in silence is all the more astonishing, because as soon as the above-mentioned Bulletin was printed I sent a proof of it to Professor Dewar ; I also forwarded him proofs of my other researches, knowing that they interest him.

“From this summary of researches, as well as of dates, it follows that the first apparatus serving to produce large quantities of the liquefied so-called permanent gases, with the solitary exception of hydrogen, was constructed by me. This apparatus can be enlarged at will by increasing its parts, but without changing anything in its construction, so that it might be used to obtain liquefied gases in factories should they at any time prove of practical utility. By means of this apparatus I obtained as large quantities of liquid gases as I wanted ; and they were used for the first time on a large scale as cooling agents (for instance, in my attempts to liquefy hydrogen), or as an object of scientific researches (the absorption spectrum of liquefied oxygen, its coefficient of refraction, &c.).

“The experiments of Professor Dewar are merely the

• repetition and confirmation of these researches, most of which were published several years before his corresponding investigations. His work is really original only as to the magnetic properties of liquid oxygen : that which is not borrowed from my researches is a development of ideas struck out by another -as, for instance, the experiments on electrical resistance at low temperatures, which were begun by Clausius, continued by Cailletet and Bouty, and brought ten years ago by my former fellow-worker, the late Professor Wroblewski, to the temperature of the freezing-point of nitrogen, then several degrees below the temperature attained in the experiments of

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