rubbing substances are arranged like a pair of mill-stones, the lower stone being a disc of iron laid horizontally, and the upper a disc of copper mounted on a vertical axis on which it can revolve. The surfaces are kept pressing against each other by a strong spring. When the upper disc is made to revolve rapidly, a very decided current is produced; and this I found to be markedly increased, as indicated by the telephone, by feeding in between the discs powdered antimony and bismuth combined. Of course we have here a series of rapid reversals of the current, as the direction of the current will depend upon whether particles of antimony or particles of bismuth are in contact with the lower plate. This clearly indicates a thermoelectric effect; and I have no doubt that the effect will be increased by applying a means whereby the upper surface of the copper plate and the lower surface of the iron one can be kept cold by a freezing mixture. As yet, however, I have not had time to try that. In another form I took two cylinders, the one of antimony and the other of bismuth, and placed them together end-wise, the pressure between them being regulated by a screw. The antimony cylinder was kept stationary, and the bismuth made to revolve very rapidly against it, so much so that both cylinders rapidly became hot. This also gave a pretty strong current.

Seeing that the friction between metals does certainly produce an electric current, it seemed natural to inquire whether an electric current sent from a battery across the surface between two metals would not modify the friction of the one against the other. I have tried to test this in a variety of ways, and the results leave me in doubt whether to attribute the indications which I have received to actual changes in the friction or to incipient fusion of portions of the surfaces together by the heat produced by the current, or to an effect similar to the Trevelyan rocker. In one experiment I made an inclined plane which carried a pair of parallel rails of copper about three quarters of an inch apart. The rails were hinged at the lower end, so that the plane could be set at any angle with the horizon. It was so arranged that the current from the battery could be sent up the one rail, through any conductor laid across the two, and down the other rail. The surfaces of the rails were made quite smooth. When a heavy piece of metal was laid across the rails, the angle of repose was the same both when there was and was not a

current passing. It was different, however, when a light body, such as a sewing needle, was put on. Then, when the current from three Bunsen cells was passing, the plane could be elevated considerably past the angle of repose for no current, before the needle rolled down. On examination I found that the needle was actually sticking to the copper; but that, in almost all cases, this sticking gave way without the angle being altered after the current had been taken off for some time, and the needle and copper allowed to come back to their normal temperature. In another experiment I employed a Bell telephone to enable me to detect any variation of friction when a current was passing between the rubbing surfaces. To the centre of the telephone disc was attached a long narrow strip of light wood; the object of making the strip so long being to remove the telephone as far as possible from the inductive action of the battery current which was to be used. To the other end of the strip was attached a flat piece of bismuth. This rested on the convex surface of a cylinder of antimony, which could be rapidly rotated. The battery current was sent through the antimony and bismuth by entering the antimony by the axis on which it revolved, and leaving the bismuth by a spring pressing tightly against it. In the battery circuit was included the violin with its microphone already mentioned, and the telephone with the rod attached was placed as the transmitting telephone in a telephone circuit. When the antimony cylinder was rapidly rotated, a listener in the receiving telephone watched attentively till his ear became accustomed to the sound produced by the rubbing, and transmitted along the wooden rod to the telephone disc. The battery circuit was then joined, and the violin played, the antimony cylinder meanwhile rotating at the same rate as before. No alteration in the sound was audible, which indicated no alteration in the friction. I then substituted a sharp point for the flat piece of bismuth, and immediately the violin sounds were faintly but clearly heard. This led me to think that some sticking was produced by the fusing of the sharp point, and more especially as the sound became a little clearer as the rotation became very slow.

Acting on this hint, it immediately occurred to me that a receiv ing telephone could be constructed depending upon this effect. I therefore took my bismuth cylinder and mounted it on a frame so that it could be made to rotate very truly on pivots. By wheels

and bands it was also made to rotate slowly. A phonograph mouthpiece, with a very thin disc of wood or mica, was next placed, so that a fine wire with a sharp point bent at a right angle, and with its other end attached to the centre of the disc just pressed with its sharp point on the convex surface of the bismuth cylinder. A current of four Bunsen cells was now passed through the wire and cylinder, and also through the violin microphone. When the violin was played the tune was heard faintly proceeding from the mouthpiece even when the bismuth cylinder was stationary. This arose simply from the loose contact of the wire and bismuth. The sound was, however, very greatly increased when the cylinder was rotated slowly, so loud indeed, that it could be distinctly heard all over an ordinary room. I have been able to transmit singing very clearly, but not speaking clearly enough to be understood. This instrument is analogous to the loud-speaking telephone of Mr Edison; but the explanation of their action must be very different if electrolysis, as is usually supposed, be the cause of the variation in the slipping of the platinum point on the chalk cylinder, which is characteristic of Edison's instrument. Quite recently the electrolytic action has been questioned, and a different explanation given by Professor Barret of Dublin. It is evident that electrolysis can in no sense come into play when the cylinder and rubbing point are both metallic. In that case two probable explanations of the action. readily suggest themselves. The one is that there is more or less of an actual sticking of the metals together, arising from their fusion by the heat of the current. If this be so, then, the loose contact is alternately made a very good one, and then one actually broken. The other is the action of the Trevelyan rocker. Here, however, we have clearly only an analogous, and not by any means an identical effect. In the Trevelyan rocker the heat passes from a large mass of hot metal through two points of contact to a cold block, whereas, in the other case the heat is only produced at the surfaces of separation, the temperature of the rest of the metals being almost unaffected. Still it appears to me that the variations of the heat at this point has a great deal to do with the actions of all microphones, and in general with all sounds transmitted from one loose contact to another. This is shown by substituting cylinders of different metals for the bismuth cylinder above mentioned, all other things remain3 Y

VOL. X.

ing the same. I have tried in this way, besides bismuth, cylinders of lead, tin, iron, antimony, and carbon, and find that bismuth gives by far the best result, In the other cases the sound from the simple loose contact is heard clearly enough; but there is hardly any increase of it produced by rotating the cylinder. Now this seems to be due in great part, if not entirely, to the difference between the metals as regards their specific heat and thermal conductivity. Obviously, with the same current, the greatest heat will be produced at the junction of the rubbing point and cylinder, when the specific heat and thermal conductivity are both as low as possible. Hence very probably the reason why bismuth answers so well, seeing that of all common metals it stands lowest on the list both in specific heat and thermal conductivity. In fact, if we take the product of the reciprocals of the specific heats and thermal conductivities of the above-mentioned metals, we find the product for bismuth greatly in excess of that for any of the others,

3. Note on the Present Outbreak of Solar Spots. By

Professor Piazzi Smyth.

4th April 1880.

The physical activity going on in the Sun is still increasing, and worthy of all admiration. There was a very large spot had come round on the North-following limb on March 29; and was after that the subject of observation from day to day as it approached the central Solar meridian. But when it arrived there, on April 3, behold two less large but still most notable spots had burst out clear and full within the previous twenty-four hours between the great spot and the preceding limb. And on this day, April 4, there are two more notable ones very close to the greatest spot, making in all five remarkable spots not only all visible at once, but working and seething positively before our eyes,

April 5, Noon. Three of yesterday's five spots are gone. Faculæ are in their place; and that is "the end of spot-life," says Prof. Alex. S. Herschel.

4. Positive and Negative Electric Discharge between a Point and a Plate and between a Ball and a Plate. By Alexander Macfarlane, M.A., D.Sc., F.R.S.E.

I have made the following observations in the Natural Philosophy classroom of the United College, St Andrews, with the view of ascertaining whether the electromotive force required to cause a spark to pass between a small globe and a plate, or between a point and a plate, differs for the two kinds of electricity. Sir William Thomson suggested that I should apply to this question the method of measuring the electromotive force required to produce sparks, which I have described in papers already contributed to the Society (Trans. Roy. Soc. Edin. vol. xxviii. p. 633). It is a problem to which Faraday attached great importance. He says at sect. 1523, vol. 1, of his Experimental Researches in Electricity: "The results connected with the different conditions of positive and negative discharge will have a far greater influence on the philosophy of electrical science than we at present imagine, especially if, as I believe, they depend on the peculiarity and degree of polarised condition which the molecules of the dielectrics concerned acquire." He records a great number of experiments on this subject in sections 1465-1525. He took sparks between a ball 0.25 inch in diameter and a ball 2 inches in diameter. When the large one was connected with a discharging train, the small one charged positively gave a much longer spark than when charged negatively; also the small ball charged negatively gave a brush more readily than when charged positively in relation to the effect produced by increasing the distance between the two balls (sect. 1489). When the interval was below 0'4 of an inch, so that the small ball gave sparks whether positive or negative, he could not, he says, observe any constant difference either in their ready occurrence or the number which passed in a given time. But when the interval was such that the small ball when negative gave a brush, then the discharges from it as separate negative brushes were far more numerous than the corresponding discharges from it when rendered positive, whether those positive discharges were as sparks or brushes (sect. 1490).* As he puts

* Drs De La Rue and Müller have found in the case of the discharge of their great chloride of silver battery that the discharge between a point