In Table II we give a summary of the separate measurements of the three coils, taken from Table I. It will be seen that the different determinations of the coils agree very closely, showing that the accidental errors of measurement are very small. The variations would

Table II.-SUMMARY OF MEASUREMENTS OF THE INDUCTANCE GIVEN IN TABLE I.

probably have been greater if the resistances had been varied, on account of variations in the resistances. The last three measurements with a little smaller current are, of course, smaller for coils F and S (serpentine spools) than the first four, but the coil C (mahogany spool) remains substantially the same throughout. In an article in Bulletin No. 2 (issued February 1, 1905), on the Absolute Measurement of Inductance, we called attention to some slight discrepancies which we were at a loss to understand. We now know that they were due to the magnetic impurities in the serpentine spools on which one of the coils was wound.

Hartmann and Braun exhibited at St. Louis some inductance coils wound on white marble. These would undoubtedly be entirely free from magnetism. Wooden spools are cheaper and when the wood is thoroughly seasoned seem to be quite permanent. This question we are investigating further. We have ordered some of these inductance standards wound on marble from Hartmann & Braun, and have designed some to be built for us in this country. When these standards have been thoroughly tested for stability and constancy with

different testing currents we shall determine their values by several different methods in order to ascertain what sensibility and how close agreement may be found among entirely different methods.

THE SILVER COULOMETER.

By K. E. GUTHE.

1. According to Faraday's law of electrolysis, a strict proportionality exists between the quantity of electricity passing through an electrolyte and the electrochemical reaction produced by it. The latter may therefore serve for the measurement of quantity of electricity. We call the instruments employed for this purpose "voltameters" or "coulometers."

A good many different types of coulometers have been used; for example, the gas coulometer, in which the volume of the liberated gases is measured, or the iodine and the iron coulometers, in which the electrochemical change is measured by titration. The usual method, however, is the determination of the mass of a substance deposited at one of the electrodes by an electric current." Of the last the copper coulometer and the silver coulometer are the best-known types.

The investigations of F. and W. Kohlrausch, Rayleigh and Sidgwick, Gray, Schuster and Crossley, and Glazebrook and Skinner, have proved that the silver coulometer is by far the most reliable instrument of this sort and that it will give results accurate to 1 in 5,000 if certain specifications as to its construction and treatment are closely followed.

a A description of some unusual types is given by Danneel: ZS. f. Electroch., 4, p. 154; 1897.

Fr. and W. Kohlrausch: Wied. Ann., 27, p. 1; 1886. Rayleigh and Sidgwick: Phil. Trans., 175, p. 111; 1884. d Gray: Phil. Mag., 22, p. 389; 1886.

Schuster and Crossley: Proc. Roy. Soc., 50, p. 344; 1892. f Glazebrook and Skinner: Phil. Trans., 183, p. 567; 1892.

2. For this reason the International Electrical Congress, held at Chicago in 1893, adopted the silver coulometer as a standard for the measurement of electric current. Though the ampere was defined as one-tenth of the unit of current of the c. g. s. system of electromagnetic units, it was added that it is represented sufficiently well for practical use by the unvarying current which, when passed through a solution of nitrate of silver in water in accordance with standard specifications, deposits silver at the rate of 0.001118 gram per second." This value is called the electrochemical equivalent of silver.

Specifications, as referred to above, were prepared by the National Academy of Sciences and legalized in the United States in 1894. They are practically in conformity with the earlier specifications of the British Board of Trade and read as follows:

In employing the silver voltameter to measure currents of about one ampère the following arrangements shall be adopted:

The cathode on which the silver is to be deposited shall take the form of a platinum bowl not less than 10 cm. in diameter and from 4 to 5 cm. in depth.

The anode shall be a disk or plate of pure silver some 30 sq. cm. in area and 2 or 3 mm. in thickness.

This shall be supported horizontally in the liquid near the top of the solution by a silver rod riveted through its center. To prevent the disintegrated silver which is formed on the anode from falling upon the cathode, the anode shall be wrapped around with pure filter paper, secured at the back by suitable folding.

The liquid shall consist of a neutral solution of pure silver nitrate containing about 15 parts, by weight, of the nitrate to 85 parts of water.

The resistance of the voltameter changes somewhat as the current passes. To prevent these changes having too great an effect on the current, some resistance besides that of the voltameter should be inserted in the circuit. The total metallic resistance of the circuit should not be less than 10 ohms.

Method of making a measurement.-The platinum bowl is to be washed consecutively with nitric acid, distilled water, and absolute alcohol. It is then to be dried at 160° C. and left to cool in a desiccator. When thoroughly cool it is to be weighed carefully. It is to be nearly filled with the solution and connected to the rest of the circuit by being placed on a clean insulated copper support to which a binding screw is attached.

The anode is then to be immersed in the solution so as to be well covered by it and supported in that position. The connections to the rest of the circuit are then to be made.

Contact is to be made at the key, noting the time. The current is to be allowed to pass for not less than half an hour and the time of breaking contact observed.

The solution is now to be removed from the bowl and the deposit washed with distilled water and left to soak for at least six hours. It is then to be rinsed successively with distilled water and absolute alcohol and dried in a hot-air bath at a temperature of about 160° C. After cooling in a desiccator it is to be weighed again. The gain in mass gives the silver deposited.

To find the time average of the current in amperes this mass, expressed in grams, must be divided by the number of seconds during which the current has passed and by 0.001118.