that the character of the propagation of electric disturbances of equilibrium is different according as the resistances to be overcome are small or great; (2) that with small resistances such as that of 120 to 400 kilometers of copper wire 2 millimeters in diameter-restoration of the equilibrium takes place by oscillations, the amplitude of which diminishes the more rapidly the greater the resistance; (3) that with great resistances-such as 1000 kilometers of the above wire-no oscillations are observed, the velocity being inversely proportional to the total resistance and to the length of the conductor, and hence, for two conductors of equal specific resistance, being inversely as the square of the length; and (4) that the velocity of electric signals magnetically produced is not, except in cases where the resistance is very large, comparable with that of disturbances of electrical equilibrium, the latter being the greater. One curious result the author draws from his investigation, i. e., that when the resistance in the circuit is very small, the motion of the electricity takes place almost exclusively upon the outer surface of the conductor.

W. Siemens has experimented to determine the velocity of electric propagation. The two outer armatures of a condenser were connected together; the two inner ones, one to the line, the other to a short wire. Both terminated in points close to a revolving smoked cylinder. On putting the arc connecting the outer armatures to earth, the condenser is discharged, and two sparks pass to the cylinder, the difference between them indicating the time of traversing the line. Siemens concludes that electricity has an actual velocity of propagation.

Sabine has proposed to use the time taken by a condenser to discharge itself, or to pass from one potential to another, through a circuit of known resistance, as a unit for measuring very small intervals of time.

Edison has discovered the fact that the conductivity of graphite loosely compressed is remarkably increased by pressure, probably from improved internal or external conHe has utilized this discovery in the construction of his talking-telephone by placing such a cylinder of graphite against the brass or mica diaphragm which receives the sound and in the main circuit. The electrical current, which

tact.

is inversely as the resistance, copies faithfully the varying pressure of the sound-waves, and transmits them along the line.

4. Electrolysis.

Ayrton and Perry have published an account of an elaborate series of experiments on ice as an electrolyte. As a result of their experiments, they state that the capacity per cubic centimeter of ice at -13.5° C. is 0.002 micro-farad, and the specific inductive capacity is 22,160 (that of air being called unity), while that of water at 8.7° is about 2240 times this amount. Commencing with ice at -13.6° C., the temperature was allowed to rise, and the conductivity determined by galvanometer readings. From these a very regular curve was deduced, which shows that the conductivity increases regularly, and that there is no sudden rise in passing from the solid to the liquid state. The same apparatus was also used to determine the electro-motive force of polarization-currents at different temperatures.

Helmholtz has published a note containing the results of a research made at his suggestion by Root to ascertain whether in galvanic polarization the electrolytic gases remained on the exterior or actually penetrated the platinum. The experiments show a very rapid penetration of the platinum, so that if for only five minutes the platinum plate experimented on and one to the right of it were connected with a Daniell's cell, a condition of polarization was developed between it and a plate to the left.

Bertrand has experimented on the electrolytic preparation of the metals, and has prepared aluminum, magnesium, cadmium, bismuth, antimony, and palladium from their aqueous solutions in this way. The current employed should be strong, and the concentration of the solution carefully regulated.

5. Electric Spark.

Spottiswoode has described the new enormous induction coil made for him by Apps, which is capable of giving sparks 42 inches long. It has two primary coils-one used for long sparks, the wire being 660 yards long and 0.096 inch diameter; the other, for fat sparks, has 84 pounds of wire, instead of 67. The secondary wire is 280 miles long, and forms 341,850 turns. In the two central sections the diameter of

this wire is 0.0095 inch, and in the two outer ones 0.0115 and 0.0110 inch. The condenser consists of 126 sheets of tin-foil 18 by 8.5 inches, separated by two thicknesses of varnished paper 0.0055 inch thick. Glass 3 inches thick has been pierced with the 28-inch spark of this coil, using five cells of Grove.

Spottiswoode also has experimented to determine the conditions of stratification in exhausted tubes, and finds that in a tube, one terminal of which is connected with the negative coating of a Leyden battery, while the other is held beyond striking distance from the positive coating, the discharge will show the separation of the positive from the negative part by a dark intervening space, and under suitable conditions of exhaustion will also show striæ. Decreasing the distance from the positive coating produced a stratified discharge. He concludes that by a suitable disposition of a Leyden battery the phenomena produced by it coincide with those produced by the induction coil.

66

Thompson has repeated and varied the experiments of Edison on induced sparks, from which the latter concluded upon the existence of an etheric" force. He has obtained these sparks ten millimeters long, and shows that they are made up of alternating currents of very short duration.

Lommel has figured two interesting electric dust figures analogous to those of Lichtenberg, but in which the rod conveying the discharge lay horizontally on the vulcanite plate instead of being vertical. One of these is positive, the other negative, and they were produced by dusting a mixture of red-lead and lycopodium powder on the plate after several sparks had passed into the rod.

Wright has studied the production of transparent metallic films by the electric discharge in exhausted tubes, and has obtained some curious results. The colors obtained by transmitted light were, for gold, brilliant green, thinning out to pinkish violet; for silver, pure deep blue; copper, dull green; bismuth, grayish blue; platinum, gray; palladium, smoky brown; lead, olive brown; zinc and cadmium, grayish blue, inclining to purple; aluminum, brownish; iron, neutral tint; nickel and cobalt, gray or brownish gray; tellurium, purple ; magnetite, gray brown. The light transmitted is powerfully polarized, the polarization increasing with the incident

angle. The metals of high atomic weight volatilize most readily.

Barat proposes to use, for the Franklin portrait experiment, in place of a gold leaf, a gilded plate of glass, having two strips of tin-foil across the ends and the portrait outline, paper and press as usual. One spark is sufficient; a second uniformly breaks the apparatus.

Gripon has published two interesting experiments in static electricity. In the first, two equal strips of copper are placed at the top of a metallic stem, the lower one fixed, the other moving on a pivot. If the apparatus be placed in the neighborhood of an electric machine, or even of a charged sphere, the two strips arrange themselves perpendicularly to one another. If the upper strip be replaced by a magnetic needle, a deflection more or less decided is also observed. In the second, a capsule completely full of oil of turpentine is placed beneath a permanently electrified sphere. The liquid is attracted, and a column of it rises to the ball, in which very complex movements may be observed, the whole recalling closely a water-spout.

Berthelot having proved that even under the ordinary electric tension of the atmosphere a silent discharge may be caused in a tube containing nitrogen, by means of which this gas may be absorbed by organic bodies to form new compounds, proposes this result as one which must necessarily go on in nature. Hence he insists upon the necessity of studying consecutively and methodically the electric condition of the atmosphere, since upon its tension this absorption of nitrogen depends.

Maxwell has communicated to the Physical Section of the British Association a paper on the protection of buildings from injury by lightning. Premising that the precautions ordinarily taken in the construction of lightning-rods are calculated rather for the benefit of the surrounding country and for the relief of clouds laboring under an accumulation of electricity than for the protection of the building on which they are erected, he goes on to advocate the protecting of a powder-mill, for instance, by sheathing its roof, walls, and ground-floor with thick sheet copper, since, under these circumstances, no electrical effect could possibly occur within it by reason of any thunder-storm outside. Ordinarily, how

ever, it would be quite sufficient to carry a No. 4 copper wire round the foundation of the house, up each of the corners and gables and along the ridges. If there are no metallic connections with distant points, such as water and gas pipes, it is not necessary to take any pains to facilitate the escape of the electricity into the earth.

Baxendell subsequently called attention to the fact that the above system of protecting buildings from lightning was suggested by the late Mr. Sturgeon in a paper read before the London Electrical Society on March 7, 1838. Mr. Sturgeon, moreover, advocated an efficient earth connection-a measure absolutely essential to prevent damage should the building be struck by lightning.

Fitzgerald has communicated to the Royal Society the important fact that a ray of plane polarized light, when reflected from the polished pole of an electro-magnet, is not simply rotated, as Kerr supposed, but emerges elliptically polarized. To account for this result, he supposes differences of density of iron in different directions due to the magnetization; whence two circular rays of unequal indices, which by their combination produce, of course, an elliptic ray.

Gordon has repeated with care the experiments of Kerr on the effect of electric charge in causing double refraction in glass, and has been entirely unable to produce the results, though the means employed were as powerful and as delicate as the latter's.

Lodge presented to the Glasgow meeting of the British Association an ingenious mechanical apparatus for illustrating many electric phenomena, such as the passage of electricity through metals, electrolytes, and dielectrics. In the apparatus the current is represented by an inelastic endless cord, the electro-motive force by a weight tending to urge the cord forward. Resistance is shown by buttons moving on the cord with friction, attached by strings more or less elastic to the supports, by means of which counter-electromotive force is represented.

6. Electric Light.

Gauduin and Gramme have experimented to determine the effect of the introduction of various more or less difficultly fusible substances into the carbons employed for the