TABLE IV. Showing the minimum current at which the gases developed visibly upon a gold-leaf partition in various electrolytes. The electrodes were platinum. The values given are usually the mean of several determinations. A 96 per cent. solution of acetic acid was found to have too high a resistance to give a sensible current with 50 volts E.M.F. Benzoic acid dissolved in the same was also an insulator. TABLE V. The Critical Current in 30 per cent. H2SO, for Aluminiumleaf, Platinum-leaf, and Gold-leaf: Table V. (continued). Influence of concentration upon the value of the Critical Current, using a Gold-leaf partition : Using a 30 per cent. solution of H2SO4, Pt partitions were used and gave the following values for the Critical Current:: To test the influence of very small holes in the specimens used as partitions, the 0.02 millim. Pt partition was punctured successively with 1, 2, 3, and 4 holes, each 0.5 millim. in diameter. This gave the following values for its Critical Current: TABLE VI. Showing the increasing Critical Current of 0.02 millim. Palladium in a 30 per cent. solution of H2SO4. A new specimen of the same palladium is being tested and is showing the same phenomenon of increase of the critical current. This last specimen was carefully selected and has no holes. Its critical current at first was 0.016; after some use it is now 0.043. I have begun to use a solution of sodium acetate with the hope that it will not oxidize the gold. I can already say that it is at least very much less active than sulphuric acid; it also shows a decided temperature coefficient for the critical current. This part of the investigation will be resumed when opportunity offers. Vanderbilt University, Nashville, Tennessee, Note added April 15, 1893. More satisfactory specimens of palladium have not yet been obtained, so the matter is left for the present with the seemingly anomalous results recorded above as I find in my note-book. As regards the temperature coefficient of the "critical current-density," I requested Mr. R. W. Clawson, a student in the laboratory, to make some measurements which are tabulated below. It may be well to state that the method of observation, that of observing the first appearance of visible gas, is not very sensitive, because this development of gas on thin partitions is not quite sudden and sharply defined, but begins gradually, especially with very thin partitions. The probable error of 66 a reading is, however, not more than 5 per cent., I think. Only "XX" gold-leaf has been used in a solution of sodium acetate. The method was to set up the voltameter with goldleaf partition, close the circuit, and run the current up till the 66 critical current-density was reached. Current and temperature were recorded, the current slightly reduced and allowed to pass till the temperature had risen, say 5 degrees, when the current was again increased to the "critical" value for this new temperature. The limit was reached at about 45 degrees C., when the sealing-wax softened. Since both "critical current-density" and conductivity of the electrolyte increase with temperature, effort was made to learn whether both these quantities have the same temperature coefficient by raising the current just to the critical value, and without altering the outside resistance to observe if the increasing current, due to increasing conductivity of the electrolyte, just sufficed to maintain an incipient escape of gas at the partition. In every case the gas soon ceased to escape on being left alone after adjusting the current to a given temperature, requiring always some reduction of the outside resistance to regain the critical current for the higher temperature. A quantitative correction for the temperature coefficient of the outside metallic resistance in the circuit would probably account for the discrepancy. TABLE VII. Temperature Coefficient of Critical Current. Sodium Acetate; sp. gr.=1.06. Gold-leaf partition. Temperature......... 22° 24° 26° 29° 30° 33° 35° Current... Current.... .65 •70 •76 80 .85 .92 •96 Temperature....... 24° 27° ⚫53 .64 26° 30° 33° 37° 40 19° 20° 23° •30 ⚫37 45 50 *65 •74 83 .96 1.92 1.15 Temperature......... 15° 17° 300 Prof. Minchin on the Coefficient of Self-Induction Table VII. (continued). New solution Sodium Acetate; sp. gr.=1.13. Temperature......... 23° 27° Current........ 70 .80 Temperature....... 14° 19° 24° 29° 34° 39° 44° Current.... .57 *67 78 .85 .93 .98 1.05 New partition. The means of all the above results plotted give almost a straight line (Plate IV. fig. 3). XXV. Calculation of the Coefficient of Self-Induction of a Circular Current of given Aperture and Cross-Section. By Professor G. M. MINCHIN, M.A.* LET ET ACB (fig. 1) represent a circular wire in which a current of strength i is circulating; let O be its centre and OV its axis (perpendicular to its plane); let P be any point in space and through P describe a circle, PQ, parallel to the plane of the current, its centre being V on the axis. It is required to calculate the normal flux of magnetic force passing through the circle PQ. If VP, and the vector potential of the current at P is G (this latter being, of course, * Communicated by the Physical Society: read December 8, 1893. |