but variable, and there is constantly nutation of the axis as well as precession, the action above described will be somewhat modified; and the elasticity of the solid material may be expected to have some influence on the result. This influence, however, will be minute, as the part of the disturbing force which is variable and produces nutation is very much smaller, even at its maximum, than the precessional force. The consideration of this matter, however, has no bearing upon the validity or not of Mr. Hopkins's method, but simply upon the numerical value of his final result, not upon the question of the fluidity or solidity of the earth's mass.

6. It will appear then, I think, to your readers that the strictures of M. Delaunay upon this method, which the genius of Mr. Hopkins devised, betray an oversight of the real point upon which the success of his method depends, and that this method stands unimpaired.

Murree, Himalayas,

I am,

Yours faithfully,

JOHN H. PRATT.

May 17, 1870.

III. On the Path of Electrical Induction- and Disjunction-Currents through Gases of various Densities, and between Poles of different shapes. By E. EDLUND*.

1. FOR

OR shortness' sake I will, in what follows, apply the term electrical disjunction-currents to those currents which have their origin in the voltaic arc or in the electrical spark, and in the same connexion will speak of the force to which they owe their origin as the electromotive force of disjunction. This name indicates that to produce these currents the conduction must be broken, in order that a luminous arc or spark may be formed, as well as that the poles between which the luminous phenomenon is formed are mechanically disintegrated +.

In the investigation I used the same electrophorus machine as that employed in my earlier experiments on these currents. An insulated copper wire, a c, is directly connected with the knob a (see the figure) on one absorber, while the insulated

* Translated from Poggendorff's Annalen for March 1870, having been read before the Swedish Royal Academy of Sciences at Stockholm, October 13, 1869.

† In the sequel I shall use the term " disintegration of the poles" to denote the whole mechanical work which the current performs in the spark, although this work is consumed not only in disintegrating the poles, but also in imparting velocity to the detached particles, in putting masses of air in motion, &c.

copper wire de terminates in a
brass knob in the vicinity of b.
From cand e insulated conduct-
ing wires pass to the knobs f
and Two other conducting
g.
wires pass from the joints c and
e to the galvanometer g, of which
instrument I have already given
a description. The bridge h,
which consists of a German-
silver wire, connects the points
i and k with one another; is a
conducting wire leading to the
earth, and m a rheostat consist-
ing of fine German-silver wire.
The deflections indicated by the
needle of the galvanometer when
the machine is in action are pro-
duced by three different cur-
rents-namely, (1) that part of
the discharge of the machine

which passes through the coil of the galvanometer, (2) the disjunction-current formed in the spark between ƒ and g, and (3) the induction-currents which are produced by the discharge in the coil of the galvanometer. As regards the first current, it is under ordinary circumstances so inconsiderable as compared with the others that it need not be taken into account. The disjunction-current, on the contrary, produces a considerable deflection; but this is very materially diminished when the induction-currents from the galvanometer traverse the spark between f and g. As I have already shown in a previous paper*, the spark acts like an electrical valve; that is, it allows one of the two equal but oppositely directed induction-currents to pass in larger quantity than the other. The difference in the two induction-currents thus occasioned acts therefore on the magnetic needle, and always in such a manner that the deflection produced by the disjunction-current is diminished. When, therefore, the magnitude of the disjunction-currents under various circumstances is to be investigated, the experiments must be so arranged that the action of the induction-currents on the magnetic needle shall be nullified as much as possible. This may be most easily effected by inserting a bridge between i and k. In this case only part of the discharge goes through the coil, and the induction is therefore

* Oefv. af Vet. Ac. Förh. 1868, p. 457; Poggendorff's Annalen, vol. cxxxvi. p. 337; Phil. Mag. S. 4. vol. xxxvii. p. 41.

weaker; moreover, of the two opposite induction-currents, equal parts pass through the bridge, by which their action on the needle is annulled. Only that part of the two currents which traverses the spark can affect the needle. In order to make this as small as possible, the resistance in the bridge must be small as compared with the sum of the resistance in the rheostat and in the spark. But in proportion as the resistance in the bridge is diminished, the deflection of the disjunction-current is also dimi nished; for this now takes its path more and more through the bridge instead of through the galvanometer. Hence the resistance in the bridge must not be made smaller than so that the action of the induction-currents upon the magnetic needle just becomes imperceptible.

Experiments specially made for this purpose showed that a German-silver wire 27 centims. in length and 0·7 in diameter fulfilled these conditions; and it was therefore used as a bridge in front of the galvanometer. That the action of the induction-currents upon the needle was thus rendered imperceptible was shown as follows:-An induction-coil, C, of exactly the same nature as the galvanometer-coil, was inserted, between m and h, and in front of it a German-silver wire as bridge, of the same length and the same diameter as the previous one. The galvanometer-coil and the coil R were thus both in the same position; they must produce equal induction-currents; and equal parts of these must traverse the respective bridges. If, then, it can be proved that the induction-currents from the coil C have no influence on the deflection of the magnetic needle when the bridge is in front of this coil, the proof is valid also in the case of the galvanometer-coil. Of the observations made the following may be adduced, with regard to which it must be observed that the resistance in the conducting-wires and in the spark was so great, compared with that in the coil and in the German-silver wire just mentioned, that the latter need not be taken into account.

Experiment 1.-The German-silver wire inserted between m and k. When the machine was put in motion the following deflections were observed:

Experiment 2.-The coil C was inserted between m and k, so that the German-silver wire formed the bridge :

Experiment 3.-To ascertain whether any change had taken place in the machine, the first experiment was repeated :

Deflections.

34.3

*

When the coil C was accompanied by the bridge, no indication of induction was observed; but when, on the contrary, the bridge was removed, the deflection was diminished by more than onehalf.

The following experiments were made in the same manner as the preceding, after the resistance in the rheostat had been doubled.

Experiment 4.-The German-silver wire in the circuit:

Experiment 5.-C inserted along with the German-silver wire, the latter as bridge to the former :

Experiment 6.-The same experiment as No. 4:

This series, therefore, gave the same result as the first.

In order to investigate the nature of the electromotive force of disjunction when the spark was formed in various more or less rarefied gases, a glass cylinder 12 centims. in length and 7 centims. in diameter was used. Brass caps could be screwed on air-tight at each end of the cylinder. În the centre of each cap was a stuffing-box, through which a round brass rod could be moved air-tight backwards and forwards. To the inner ends of these brass rods were firmly screwed the poles which were used in the experiments. The outer ends were provided with bindingscrews for the conducting-wires; and on one rod was marked a scale of millimetres, to determine the distance between the poles. The brass rods were insulated from the caps and the stuffing-boxes. Phil. Mag. S. 4. Vol. 40. No. 264. July 1870.

C

On one of these caps was screwed a brass tube with a stopcock, the other end of which was provided with a screw-thread by which it could be screwed to an air-pump. The brass tube was bent at right angles, so that during the experiments the glass cylinder was in a horizontal position.

To convince myself whether the bridge acted efficiently when the spark was formed in rarefied air, the glass cylinder was inserted between the points c and e, and the air rarefied until it was at a pressure of 15 millims.; the spark, therefore, at fg was formed in rarefied air.

Experiment 7.-The German-silver wire in the circuit:

Experiment 8.-C inserted along with the German-silver wire, the latter as bridge to the former :

Hence a distinct action of the induction-currents could not be observed; and the same result was obtained when the air was exhausted to a pressure of 6 millims. In all the subsequent experiments the bridge remained in the same position in front of the galvanometer; and thus the deflections obtained were independent of the induction-current of the galvanometer-coil. When in the sequel nothing is said to the contrary, the poles consisted of two equal-sized brass knobs.

2. Comparison between dry air and air saturated with aqueous

vapour.

The air was dried before entering the cylinder by being passed slowly through two glass vessels which were filled with pieces of pumice impregnated with concentrated sulphuric acid, and then through a tube filled with chloride of calcium. The air was moistened by having to pass through a long glass tube which contained pieces of wetted paper.