« PreviousContinue »
trode and thus setting up a chemical action foreign to that of the cell proper.
In the cell under notice, this line of demarcation is produced, as already stated, at the exit of the branch H, and can always be reproduced or restored to the requisite definition, if destroyed, by opening the cock at H and drawing off a portion of the combined liquids at that point, the levels in their respective limbs A B being maintained constant by a further supply from the reservoirs CD.
The operation of filling the tubes in the first instance must be systematically carried into effect.
To ensure success, the experimenter must proceed as follows : The cock controlling the reservoir C is opened, and the whole U-tube filled with the denser zinc sulphate solution; the zinc electrode, with its concomitant air-tight stopper, is next inserted in position in the upper extremity of limb A. The cock controlling branch His then opened, and, in consequence, the level of the solution in B falls, whilst that in A remains constant; simultaneously the cock of reservoir D is opened, thus allowing the copper sulphate solution to flow gently down limb B and replace the zinc sulphate solution flowing out at H. When the line of demarcation between the two liquids reaches the level of H, all cocks are closed, and the copper electrode and its stopper inserted in position.
To ensure accuracy the copper electrode should be lightly coated by electro-deposition with a film of new copper, immediately before use.
There are two strengths of solutions usually employed in this cell. One consists in making both the zinc and copper sulphate solutions to an equal specific gravity of 1.2 at 15 deg. C., the other consists of a zinc sulphate solution of specific gravity 1.4 produced by dissolving 55.5 parts by weight of zinc sulphate in 44.5 parts by weight of distilled water, and a copper sulphate solution of specific gravity 1.1, produced by dissolving 16.5 parts by weight of copper sulphate in 83.5 parts by weight of distilled water, both operations being performed at 15 deg. C.
The resultant E.M.F. varies with the solutions employed, and is 1.102 volts with the former, and 1.072 volts with the latter.
The temperature variation in E.M.F. of this cell is .00015 volt (-) per 1 deg. O. (+).
The drawbacks to this type of cell as a standard lie in its unsuitability for transport unless empty, which fact renders it more or less a laboratory instrument, and also in the fact that minute variations in both the electrodes and the electrolyte are productive of appreciable errors in the resultant electromotive force of the cell.
Grotian's Form of Standard Daniell Cell deserves mention in that it is ingenious, easily constructed, and will stand casual short circuiting without appreciable
disturbance of its constancy, three qualifications which, in themselves, constitute the value of this form of cell as a standard.
Its principle will be understood on reference to Fig. 22, in which A and B are two separate containing_vessels, in which are placed vertically the electrodes Zn and Cu, consisting respectively of pure amalgamated zinc and electrolytic copper. They are held in place by suitable grooves or supports in the containing vessels, and are immersed in their respective solutions of zinc sulphate sp. gr. 1.2, and copper sulphate sp. gr. 1.1. are projecting lips moulded on the edges of the containing vessels, and over the surfaces of which are spread the strips of filter paper indicated by the thickened lines in the figure. These strips are immersed in the two liquids to their full depth, and their free extremities project beyond the two lips as shown, below the level of the liquids in the vessels A and B. Acting by capil
lary attraction, the strips of filter paper convey the two liquids over to their free extremities, which become saturated, and the action of the cell is started by pressing the two free extremities together with the fingers until they adhere of their own accord. A gradual syphoning of the respective solutions goes on, the excess of moisture finding its escape in the form of drops, thus constantly exposing fresh surfaces of the solutions to one another. The E.M.F. of this cell is 1.101 volt, rising gradually by .001 to .002 volt per 18 hours when standing idle. Its internal resistance runs into some thousands of ohms, but, as before stated, it will stand shortcircuiting without appreciable alteration of the subse quent E.M.F.
There is no tangible standard of current, the practical unit being the ampère, but in this connection we must recall Ohm's familiar law, which has it that the current in any circuit is equivalent to the E.M.F. in that circuit divided by its resistance, the quantities named being in terms of their respective units the ampère, volt, and
E ohm, or, as it is commonly expressed, C =
R The ampère or unit of current is that current which flows through a resistance of one ohm, under the influence of an E.M.F. of one volt. In cases where exceedingly small currents have to be dealt with, as, for instance, in telephone work, a much smaller quantity is called into requisition, viz., the milliampère, which is equivalent to one-thousandth part of an ampère.
The unit of electrical resistance is the ohm; it is the resistance offered to an unvarying electric current by a column of mercury of a constant cross-sectional area of one square millimetre, and of a length of 106.3 centimetres at the temperature of melting ice.
In practice we have also the microhm, or one-millionth part of an ohm, and also more frequently the megohm, which is equivalent to 1,000,000 ohms. This latter quantity is chiefly used when expressing the resistance of dielectrics, or, in other words, the “insulation resistance" of an object.
The ohm and megohm are usually denoted by the Greek letters (w) and (n) respectively; thus 2 w means two ohms, and 2 a indicates two megohms, or two million ohms. As these symbols will be adopted where necessary in the following context, it will be advisable for the reader to impress them upon his memory.
The standard ohm for practical purposes is usually made up in the form illustrated below. It consists of an accurately calibrated coil ending in the two stout ter
Standard One-Ohm Resistance Coil, by Elliott Bros. minals depicted in the illustration. These take the form of brass or copper rods, the outer extremities of which are brought into connection with concomitant apparatus by means of mercury cups.
The coil is embedded in paraffin wax, and the brass-containing case may be immersed in a water bath up to a certain height, in order to bring it to an even temperature, which is recorded by means of a suitable thermometer also immersed in the bath, and the necessary temperature corrections applied to the subsequent results.
In Professor Chrystal's type of standard, a thermoelectric couple is added to the apparatus, one junction of which is inside, and the other outside the containing case. The couple is connected by a convenient pair of terminals to a low resistance galvanometer; if the passage of a current be indicated, the temperature of the box and the surrounding air are not synonymous, and the necessary precautions for variable temperatures must be taken; if, however, no current be indicated by the galvanometer, the exterior and interior temperatures are the same, a fact which simplifies the modus operandi.
Dr. Fleming has devised an improved form of standard resistance coil which is shown in the accompanying illustration. The resistance itself consists of a length of platinum silver wire, treble silk-covered and ozokerited, enclosed in a circular square-sectioned trough, procured by bolting two channel-shaped rings together, a hermetical joint being ensured by the use of rubber packing. Connection to the coil is obtained by the usual stout brass or copper rods, which pass down, and are insulated from the brass tubes shown in the illustration. These tubes
Standard One-Ohm Resistance Coil, designed by Dr. Fleming,
and manufactured by Elliott Bros. are insulated at the base, where they enter the ring, by an ebonite collar, and at the apex by funnels of the same material, with corrugations in their outer surfaces. Extra insulation is attained, if necessary, by pouring insulating oil into the funnels.
The advantages of this type lie in the possibility of total immersion of the ring in a water bath, together with the extra radiation of heat afforded by the extensive area of the ring itself.
The actual unit of electrostatic capacity is the farad, a quantity which, however, is seldom, if ever, employed in practice, owing to the enormity of its dimensions. It is that capacity which, when charged by an ampère of current enduring for one second of time, possesses an E.M.F. of one volt.
The practical unit adopted is the microfarad, which is equivalent to one-millionth part of a farad, and even this quantity is usually split up into three parts, one of