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Coefficient of Expansion.

As the observations on the coefficient of expansion had to be made with the same pieces of palladium used in the former experiments, the only method that could be employed was to weigh the hydride in distilled water of different temperatures, as Matthiessen did in his well-known paper "On the Expansion of Metals and Alloys" (Phil. Trans. 1866), and to deduce the mean cubical expansion from the difference of weights and the known density of water. Experiments made in this way require great care in execution, and, when every precaution is taken, are yet liable to considerable variation. Any difference of temperature in different portions of the water in which the alloy is weighed at the time of the observation, causing currents in the fluid, or the condensation of moisture on the fine platinum wire used to suspend the substance, renders the results useless. But in this case the difficulties are greatly increased from the minute bubbles of hydrogen that are apt to accumulate at any angular point of the mass, and must be removed by suddenly depressing or lifting the mass. After making a great many observations in this way, it became clear that the method would not yield results of very great accuracy when applied to the case of palladium containing hydrogen; in the mean time the results obtained are provisionally stated. Generally a mass of palladium containing hydrogen nearly equivalent to the atomic proportion Pds H2, yields the following values for the mean coefficient of cubical expansion:

Between 8 and 50

Between 0 and 80

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The cubical expansion of palladium being 0.000033, we may say the alloy with hydrogen is just twice as great. If the expansion of Graham's alloy is assumed to be equal to the sum of the expansion of the respective volumes of the constituents, then the calculated result for the cubical expansion of hydrogen is 0.000246, a number about one and a half times the coefficient of expansion of mercury.

With a fused mass of palladium containing a small charge of hydrogen, the coefficient of expansion was found to be 0.00048, and the calculated value for the occluded hydrogen became then 0.00059.

Absorption of Hydrogen at a Red Heat,

Graham's theory of the rapid passage of hydrogen through palladium at high temperatures, assumes at first a direct absorption of the gas, and then a transmission of it by a kind of "cementation process." That an absorption of hydrogen takes

place with its attendant increase of volume at high temperatures may be shown as follows:

Take a strip of thin sheet palladium, 4 or 5 centims. long and about 5 millims. in breadth, clamp it firmly by the end in a suitable support so that the strip is free to vibrate, and insert it edgeways in the middle of a hydrogen-flame burning from a nozzle about a millimetre in diameter. If the palladium is now depressed into the inner dark cone, it immediately begins to vibrate, producing a low musical note.

If the flame be extinguished by stopping the current of hydrogen for an instant, or allowing the gas to flow, the vibration commences again, and may be kept up without any actual flame.

The motion in this position in the flame is due to the absorption of hydrogen on the cool side next the inner cone, with its attendant increase of length, producing a bending of the sheet into the hot portion of the flame, where the hydrogen is instantly expelled from the palladium, which is forced to return to its original position from its natural elasticity.

The experiments detailed in this paper have been made with palladium very handsomely placed at my disposal by Messrs. Johnson, Matthey, and Co. of Hatton Garden, London; and I gladly avail myself of this opportunity of thanking them for the means of conducting this research.

XLII. Determination of the Absolute Value of the Siemens Mercury Unit of Electrical Resistance. By F. KOHLRAUSCH. [With a Plate.] [Continued from p. 309.]

FOR

V. Data of Observation.

OR determining the three fundamental magnitudes, length, mass, and time, there were used the original platinum metre belonging to the Cabinet for Metals and Machinery in Göttingen (and kindly lent for the purpose by Professor Ulrich), a Fortin's set of platinum weights belonging to the Physical Institute, and the normal clock of the Astronomical Observatory (by which that in the Physical Observatory was regulated). Oertling's comparator, which formed the basis of all the measurements, was compared with the platinum metre. It was found that the parts of this bar at -8° were equal to the divisions of the platinum metre at 0°.

For measuring the larger distances, a wooden rod 5 metres in length, divided into centimetres and provided with a slider divided into millimetres, was used. The divisions were afterwards compared with the Oertling. This was also the case with the divisions of the paper scale which was used for observation.

Intensity of Terrestrial Magnetism.

Professor Klinkerfues had the goodness to arrange that, during the period of the observations, local magnetic influences should be avoided in the Astronomical Observatory where the variation-instruments were suspended. For observing the latter I am indebted to Dr. Riecke.

With regard to the arrangement of the bifilar magnetometer, together with accessory needle, I refer to Weber's article, "Determination of the rectangular component of the Earth's Magnetic Force in Göttingen, 1834-1853"*. The value of a division of the scale of the bifilar magnetometer at that time I found to be equal to 0.000105 in parts of the whole. The intensity corresponding to a position & of the bifilar will be obtained in the sequel from the expression

T=1.83846 (1+0.000105.8).

Absolute measurement.-The moment of inertia of the principal bar was, according to a new determination which agreed to within Toooo with one formerly made by Weber,

K=42997.106 millimetre? milligramme,

the temperature in two successive observations being 15°. The ratio of torsion was 0.01085.

The time of vibration, e, observed with the arc of p divisions, gives that reduced to zero, since the scale was at a distance of 4125 divisions from the mirror,

7(1 –

p2

256.41252

)=7(1−0·00000000023.p2).

If, further, during the observations of vibration & was the mean position of the bifilar magnetometer calculated from a definite point of the scale (to which all the observations are reduced), the time of vibration actually observed must be multiplied with 1+.0·000105.8; we therefore put

To=7(1-0·00000000023.p2+0·000052.8).

Let, finally, M denote the magnetism which the vibrating bar possesses of itself—that is, in the east-west direction in which it was afterwards used as deflecting bar. In the vibrations M has a magnetism of position, which Weber had determined in the case of this bar to be equal to 780000 absolute units. Then

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For the purpose of making observations of deflection, the bar

* Abh. d. Gött. Ges. 1855, vol. vi.

M, without being displaced, was fixed in a rotating clamp, so that it could be adjusted in an east and west direction. It thereby acts upon a smaller needle (accessory needle) with a inirror, which is suspended north and south to strings which hang from a long brass rod to the top of the room. Half of the distance of these from each other, the temperature being perceptibly the same in the two determinations, was determined once for all, and was found to be

R=1501.70 millimetres.

1

The correction-term with R in consequence of the ratio of the length 1:2 of the principal bar to the accessory needle, is brought almost to zero. By deflections from second distances it was determined that the angle of deflection & at the distance R was expressed by

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The distance of the mirror from the scale, the position of which was always given by the unused string, amounted to 3015 millimetres.

Ratio of torsion of the accessory needle =0·00241.

If, finally, the mean position of the bifilar during the deflections be denoted by 8, we may put for the observed angle of deflection

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Determinations of intensity were made on the 19th and 22nd of August. The duration of the vibration was each time observed at the beginning and at the end. There were obtained

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The observations of deflections of the accessory needle, upon which the bar M acted in the two east-westerly positions at an angle of 180° with each other, gave on a scale, the centre of which was at 770 millims. :

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We find from this for the zero-point of the bifilar magnetometer the values mentioned above (page 306),

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(The magnetism M of the principal bar is found in close agreement, 538630000 or 538710000.)

=

From the degree of agreement of all these numbers, it seems to result that, with such means, a determination of the horizontal intensity to within an error of at most 0·1 per cent. may be safely undertaken.

Determination of Absolute Resistance.

I shall always collate the measurements of one kind. In so far as refers to operations which belong to any single operation out of the four, they will be discriminated by Ia., Ib., II., and III.

Terrestrial Inductor.-This has been described in Weber's paper, "Application of Magnetic Induction to the Measurement of Inclination" (Abh. der Gött. Ges. 1853, vol. v. p. 53). The surface of the coil is there given at 39216930 square millims. From a comparison of the standard then used with the normal metre, one part of the former is equal to 1.00086 millim.; so that the above number is changed into

S=39284000 square millims.

The galvanoscope (Pl. IV. fig. 1 in of the true size) consists of a multiplier of about 250 coils of copper wire 3 millims. in diameter, in 10 coils, on a wooden frame 100 millims. in breadth. The cylindrical magnets of the astatic couple are each 170 mil

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