Electro-magnetic dimensional formula, L -1 MT. Absolute unit, one c. G. s line per sq centimetre. In practice, excluding the earth's field, intensities range from 100 to 20,000 lines per sq. cm., and the working unit would perhaps have the prefix milli- or micro-. 4th. Magnetic Reluctance. Definition.-Unit reluctance in a magnetic circuit permits unit magnetic flux to traverse it under the action of unit magneto-motive force. -1 Dimensional formula, L M° T”. The practical unit is 10-9 the absolute unit. Reluctances vary in present practical work from 100,000 to 100,000,000 of these practical units, so that the working unit would perhaps employ the prefix mega-. A. E. KENNELLY, Chairman. WM. E. GEYER, GEO. A. HAMILTON, GEO. B. PRESCOTT, JR. APPENDIX III. The following specifications are those contained in the recent report of the British Board of Trade Committee on Electrical Standards: THE SILVER VOLTAMETER. 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 one 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 cm. in diameter, and from 4 cm. to 5 cm. in depth. The anode should be a plate of pure silver, some 30 square cm. in area, and two or three millimetres 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 around 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 voltmeter should be inserted in the circuit. The total metallic resistance of the circuit should not be less than 10 ohms. Method of Making Measurement. The platinum bowl is washed with nitric acid and distilled water, dried by heat, and then left to cool in a desiccator. When thoroughly dry, it is weighed carefully. It is nearly filled with the solution, and connected to the rest of the circuit by being placed on a clean copper support, to which a binding-screw is attached. This copper support must be insulated. The anode is then 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 made. Contact is made at the key, noting the time of contact. The current is allowed to pass for not less than half an hour, and the time at which contact is broken is observed. Care must be taken that the clock used, is keeping correct time during this interval. The solution is now remove from the bowl, and the deposit is washed with distilled water, an eft to soak for at least six hours. It is then rinsed successively with distilled water and absolute alcohol, and dried in a hot-air bath, at a temperature of about 160 deg. C. After cooling in a desiccator, it is weighed again. The gain in weight gives the silver deposited. To find the current in amperes, this weight, expressed in grammes, must be divided by the number of seconds during which the current has been passed, and by .001118. The result will be the time-average of the current, if during the interval the current has varied. 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 taken at frequent observed intervals of time. These observations give a curve from which the reading corresponding to the main current-time-average of the current -can be found. The current, as calculated by the voltameter, corresponds to this reading. THE CLARK CELL. Definition of the Cell.-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. 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, 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 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. (See notes below.) The crystals should be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not exceed 30 deg. C. Mercurous sulphate treated as described in paragraph 4 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. 4. 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 pro cess at least twice. (See notes below.) After the last washing drain off as much of the water as possible. Mix the washed mercurous sulphate with the zine 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 deg. 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 ensures 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 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. To Set Up the Cel'.-The cell may conveniently be set up in a small test tube of about 2 cm. diameter, and 6 cm. or 7 cm. deep. Place the mercury in the bottom of this tube, filling it to a depth of say 1.5 cm. Cut a cork about .5 cm. 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 some hours before use. Pass the zinc rod about 1 cm. through the cork. 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 2 cm. 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 24 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 should project above the top of the marine glue. 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 sur face 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. NOTES. The Zinc Sulphate Solution.-The object to be attained is the preparation of a neutral solution of pure zinc sulphate saturated with ZnSO, 7 H2O. At temperatures above 30 deg. C., the zine sulphate may crystallize out in another form; to avoid this, 30 deg. C. should be the upper limit of temperature. At this temperature, water will dissolve about 1.9 times its weight of the crystals. If any of the crystals put in, remain undissolved, they will be removed by the filtration. The amount of zinc oxide required, depends on the acidity of the solution, but two per cent. will, in all cases which will arise in practice with reasonably good zinc sulphate, be ample. Another rule would be to add the zine oxide gradually, until the solution became slightly milky. The solution, when put into the cell, should not contain any free zinc oxide; if it does then, when mixed with the mercurous sulphate, zinc sulphate and mercurous oxide are formed; the latter may be deposited on the zinc and affect the E. M. F. of the cell. The difficulty is avoided by adding, as described, about 12 per cent. of mercurous sulphate before filtration; this is more than sufficient to combine with the whole of the zinc oxide originally put in, if it all remains free; the mercurous oxide formed, together with any undissolved mercurous sulphate, is removed by the filtration. The Mercurous Sulphate.-The treatment of the mercurous sulphate has for its object the removal of any mercuric sulphate which is often present as an impurity. Mercuric sulphate decomposes in the presence of water into an acid and a basic sulphate. The latter is a yellow substance-turpeth mineral-practically insoluble in water; its presence, at any rate, in moderate quantities has no effect on the cell. If, however, it is formed, the acid sulphate is formed also. This is soluble in water, and the acid produced affects the E. M. F. The object of the washings is to dissolve and remove this acid sulphate, and for this purpose the three washings described in the specification will, in nearly all cases, suffice. If, however, a great deal of the turpeth mineral is formed, it shows that there is a great deal of the acid sulphate present, and it will then be wiser to obtain a fresh sample of mercurous sulphate, rather than try by repeated washings to get rid of all the acid. The free mercury helps in the process of removing the acid, for the acid mercuric sulphate attacks it, forming mercurous sulphate and acid, which is washed away. The cell may be sealed in a more permanent manner by coating the marine glue, when it it set, with a solution of sodium silicate, and leaving it to harden. |