In determining by this method the constant of an instrument the current should be kept as nearly constant as possible, and the readings of the instrument observed at frequent intervals of time. These observations give a curve from which the reading corresponding to the mean current (time average of the current) can be found. The current, as calculated by the voltameter, corresponds to this reading.

SPECIFICATION B.-On the preparation of the Clark cell.

DEFINITION OF THE CELL.

The cell consists of zinc, or an amalgam of zinc with mercury, and of mercury in a neutral saturated solution of zinc sulphate and mercurous sulphate in water, prepared with mercurous sulphate in excess.

PREPARATION OF THE MATERIALS.

1. The mercury.-To secure purity it should be first treated with acid in the usual manner and subsequently distilled in vacuo.

2. The zinc.-Take a portion of a rod of pure redistilled zinc, solder to one end a piece of copper wire, clean the whole with glass paper or a steel burnisher, carefully removing any loose pieces of the zinc. Just before making up the cell dip the zinc into dilute sulphuric acid, wash with distilled water, and dry with a clean cloth or filter paper.

3. The mercurous sulphate.—Take mercurous sulphate, purchased as pure, mix with it a small quantity of pure mercury, and wash the whole thoroughly with cold distilled water by agitation in a bottle; drain off the water, and repeat the process at least twice. After the last washing, drain off as much of the water as possible.

4. The zinc sulphate solution.-Prepare a neutral saturated solution of pure ("pure recrystallized") zinc sulphate by mixing in a flask distilled water with nearly twice its weight of crystals of pure zinc sulphate, and adding zinc oxide in the proportion of about 2 per cent by weight of the zinc sulphate crystals to neutralize any free acid. The crystals should be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not exceed 30° C. Mercurous sulphate treated as described in 3 should be added in the proportion of about 12 per cent by weight of the zine sulphate crystals to neutralize any free zinc oxide remaining, and the solution filtered, while still warm, into a stock bottle. Crystals should form as it cools.

5. The mercurous sulphate and zinc sulphate paste.-Mix the washed mercurous sulphate with the zinc sulphate solution, adding sufficient crystals of zinc sulphate from the stock bottle to insure saturation, and a small quantity of pure mercury. Shake these up well together to form a paste of the consistence of cream. Heat the paste, but not above a temperature of 30° C. Keep the paste for an hour at this temperature, agitating it from time to time, then allow it to cool; continue to shake it occasionally while it is cooling. Crystals of zinc sulphate should then be distinctly visible, and should be distributed throughout the mass; if this is not the case, add more crystals from the stock bottle and repeat the whole process.

This method insures the formation of a saturated solution of zinc and mercurous sulphates in water.

TO SET UP THE CELL.

The cell may conveniently be set up in a small test tube of about 2 centimeters diameter and 4 or 5 centimeters deep. Place the mercury in the bottom of this tube, filling it to a depth of, say, 0.5 centimeter. Cut a cork about 0.5 centimeter thick to fit the tube; at one side of the cork bore a hole through which the zinc rod can

pass tightly; at the other side bore another hole for the glass tube which covers the platinum wire; at the edge of the cork cut a nick through which the air can pass when the cork is pushed into the tube. Wash the cork thoroughly with warm water, and leave it to soak in water for some hours before use. Pass the zinc rod about

1 centimeter through the cork.

Contact is made with the mercury by means of a platinum wire about No. 22 gauge. This is protected from contact with the other materials of the cell by being sealed into a glass tube. The ends of the wire project from the ends of the tube; one end forms the terminal, the other end and a portion of the glass tube dip into the mercury.

Clean the glass tube and platinum wire carefully, then heat the exposed end of the platinum red-hot, and insert it in the mercury in the test tube, taking care that the whole of the exposed platinum is covered.

Shake up the paste and introduce it without contact with the upper part of the walls of the test tube, filling the tube above the mercury to a depth of rather more than 1 centimeter.

Then insert the cork and zinc rod, passing the glass tube through the hole prepared for it. Push the cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be nearly all expelled, and the cell should be left in this condition for at least twenty-four hours before sealing, which should be done as follows:

Melt some marine glue until it is fluid enough to pour by its own weight, and pour it into the test tube above the cork, using sufficient to cover completely the zinc and soldering. The glass tube containing the platinum wire should project some way above the top of the marine glue.

The cell may be sealed in a more permanent manner by coating the marine glue, when it is set, with a solution of sodium silicate, and leaving it to harden.

The cell thus set up may be mounted in any desirable manner. It is convenient to arrange the mounting so that the cell may be immersed in a water bath up to the level of, say, the upper surface of the cork. Its temperature can then be determined more accurately than is possible when the cell is in air.

In using the cell sudden variations of temperature should as far as possible be avoided.

The form of the vessel containing the cell may be varied. In the H form the zinc is replaced by an amalgam of 10 parts by weight of zinc to 90 of mercury. The other materials should be prepared as already described. Contact is made with the amalgam in one leg of the cell, and with the mercury in the other, by means of platinum wires sealed through the glass.

LEGAL DEFINITION OF THE ELECTRICAL UNITS IN

CANADA.

[Assented to July 23, 1894, 57-58 Vict., c. 38.]

Her Majesty, by and with the advice and consent of the Senate and House of Commons of Canada, enacts as follows:

1. This act may be cited as The Electrical Units Act.

2. The units of electrical measure for Canada shall be the following:

(a) As a unit of resistance, the ohm, which is based upon the ohm equal to 10o units of resistance of the centimeter-gramme-second system of electro-magnetic units, and is represented by the resistance offered

Ohm.

to an unvarying electric current by a column of mercury, at the temperature of melting ice 14.4521 grammes in mass, of a constant cross-sectional area and of the length of 106.3 centimeters.

(b) As a unit of current, the ampere, which is one-tenth of the unit of current of the centimeter-gramme-second system of electro-magnetic units, Ampere. and is represented sufficiently well for practical use by the unvarying current, which, when passed through a solution of nitrate of silver in water, and in accordance with the specification contained in schedule 1 to this act, deposits silver at the rate of 0.001118 of a gramme per second.

Volt.

(c) As a unit of electromotive force, the volt, which is the electromotive force that, steadily applied to a conductor whose resistance is 1 ohm, will produce a current of 1 ampere, and which is represented sufficiently well for practical use by 1999 of the electromotive force between the poles or electrodes of the voltaic cell known as Clark's cell, at a temperature of 15° centigrade and prepared in accordance with the specification contained in schedule 2 to this act.

Coulomb.

Farad.

(d) As a unit of quantity, the coulomb, which is the quantity of electricity transferred by a current of 1 ampere in one second. (e) As a unit of capacity, the farad, which is the capacity of a condenser charged to a potential of 1 volt by 1 coulomb. (ƒ) As a unit of work, the joule, which is equal to 107 units of work in the centimeter-gramme-second system, and is represented sufficiently well for practical use by the energy expended in one second by 1

Joule.

ampere in 1 ohm.

(g) As a unit of power, the watt, which is equal to 10 units of power in the centimeter-gramme-second system, and is represented sufficiently well for practical use by the work done at the rate of 1 joule per

Watt.

second.

(h) As a unit of induction, the henry, which is the induction in a circuit when the electro-motive force induced in that circuit is 1 volt, while the Henry. inducing current varies at the rate of 1 ampere per second.

3. The units of electrical measure described in the next preceding section, or such standard apparatus as is necessary to produce them, shall be deposited in the Department of Inland Revenue and shall form part of the system of standards of measure and weight established by The Weights and Measures Act.

SCHEDULE ONE.

In the following specification, the term silver voltameter means the arrangement of apparatus by means of which an electric current is passed through a solution of nitrate of silver in water. The silver voltameter measures the total electrical quantity which has passed during the time of the experiment; and by noting this time, the time average of the current, or, if the current has been kept constant, the current itself, can be deduced.

In employing the silver voltameter to measure currents of about 1 ampere, the following arrangements should be adopted: The cathode on which the silver is to be deposited should take the form of a platinum bowl not less than 10 centimeters in diameter and from 4 to 5 centimeters in depth. The anode should be a plate of pure silver 30 square centimeters in area and 2 or 3 millimeters in thickness. This is supported horizontally in the liquid near the top of the solution by a platinum wire passed through holes in the plate at opposite corners. To prevent the disintegrated silver which is formed on the anode from falling onto the cathode, the

anode should be wrapped round with pure filter paper, secured at the back with sealing wax.

The liquid should 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.

SCHEDULE TWO.

The cell consists of zinc and mercury in a saturated solution of zinc sulphate and mercurous sulphate in water, prepared with mercurous sulphate in excess, and is conveniently contained in a cylindrical glass vessel.

The mercury.-To secure purity it should be first treated with acid in the usual manner, and subsequently distilled in vacuo.

The zinc.-Take a portion of a rod of pure redistilled zinc, solder to one end a piece of copper wire, clean the whole with glass paper, carefully removing any loose pieces of the zinc. Just before making up the cell dip the zinc into dilute sulphuric acid, wash with distilled water, and dry with a clean cloth or filter paper.

The zinc sulphate solution.-Prepare a saturated solution of pure ("pure recrystallized") zinc sulphate by mixing in a flask distilled water with nearly twice its weight of crystals of pure zinc sulphate, and adding zinc oxide in the proportion of about 2 per cent by weight of the zinc sulphate crystals to neutralize any free acid. The crystals should be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not exceed 30° C. Mercurous sulphate treated as hereinafter described should be added in the proportion of about 12 per cent by weight of the zinc sulphate crystals and the solution filtered, while still warm, into a stock bottle. Crystals should form as it cools.

The mercurous sulphate.-Take mercurous sulphate, purchased as pure, and wash it thoroughly with cold distilled water by agitation in a bottle; drain off the water, and repeat the process at least twice. After the last washing, drain off as much of the water as possible.

Mix the washed mercurous sulphate with the zinc-sulphate solution, adding sufficient crystals of zinc sulphate from the stock bottle to insure saturation, and a small quantity of pure mercury. Shake these up well together to form a paste of the consistence of cream. Heat the paste, but not above a temperature of 30° C. Keep the paste for an hour at this temperature, agitating it from time to time, then allow it to cool, continuing to shake it occasionally while cooling. Crystals of zinc sulphate should then be distinctly visible, and should be distributed throughout the mass. If this is not the case, add more crystals from the stock bottle, and repeat the whole process. This method insures the formation of a saturated solution of zinc and mercurous sulphates in water.

Contact is made with the mercury by means of a platinum wire about No. 22 gauge. This is protected from contact with the other materials of the cell by being sealed in a glass tube. The ends of the wire project from the ends of the tube; one end forms the terminal; the other end and a portion of the glass tube dip into the mercury.

LEGAL DEFINITION OF THE ELECTRICAL UNITS IN

GERMANY.

[Law of June 1, 1898, R. G. Bl., p. 905.]

SECTION 1. The legal units for electrical measurements are the ohm, ampere, and

volt.

Ohm.

SEC. 2. The unit of electrical resistance is the ohm. It is represented by the resistance of a column of mercury, at the temperature of melting ice, of uniform cross section, practically equivalent to 1 square millimeter, of a length of 106.3 centimeters, and of a mass of 14.4521 grammes. SEC. 3. The unit of current is the ampere. It is represented by the unvarying electric current which in passing through an aqueous solution of Ampere. silver nitrate deposits in one second 0.001118 grammes of silver. SEC. 4. The unit of electro-motive force is the volt. It is represented by the electro-motive force which when applied to a conductor having a resistance of 1 ohm produces a current of 1 ampere.

Volt.

SEC. 5. The Bundesrath is empowered

(a) To fix the conditions under which the silver is to be deposited, in the definition of the ampere. (Sec. 3.)

(b) To fix designations for the units of electric quantity, energy, power, capacity, and inductance.

(c) To prescribe designations for the multiples and submultiples of the electrical units.

(d) To fix the manner in which the strength, electro-motive force, energy, and power of alternating currents is to be calculated.

SEC. 6. According to this paragraph instruments used in the measurement of electrical power for commercial purposes must have their indications based on the legal units. The use of incorrect measuring instruments is prohibited. The Bundesrath is empowered to fix the limits of tolerance for such apparatus, after giving a hearing to the Physikalisch-Technische Reichsanstalt.

The Bundesrath is empowered to issue regulations concerning the official verification and periodic reverification of measuring apparatus.

SEC. 7. The Physikalisch-Technische Reichsanstalt is directed to construct primary mercurial resistance standards and assume responsibility for their control and safe custody at different places. It is also to reverify the resistance of standards of solid metals used in the intercomparisons by an annual recomparison with the mercurial standards.

SEC. 8. The Physikalisch-Technische Reichsanstalt is to provide for the issue of officially certified standard resistances and standard cells for the measurement of current and electro-motive force.

SEC. 9. The official testing and certification of electrical measuring instruments shall be carried out by the Physikalisch-Technische Reichsanstalt. The Imperial Chancellor may intrust this authority elsewhere. All standards and measuring apparatus employed in official verifications must be certified to by the PhysikalischTechnische Reichsanstalt.

SEC. 10. The Physikalisch-Technische Reichsanstalt is to assume the responsibility that the official testing and verification of electrical measuring apparatus in the German Empire shall be made in a uniform manner. It is to assume the technical supervision of the inspection service, and to prescribe all technical specifications concerning the same. It is especially to determine what kind of measuring instru