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which serves for the actual standard, which is thus usually given a value of one-third microfarad.

The capacity standard or “condenser," as it is technically termed, is commonly made up in the form illustrated. It consists of sheets of tinfoil sandwiched be tween sheets of shellac-varnished mica of a slightly larger area than the foil. Alternate sheets of the foil are connected together comb fashion, and to the two massive terminal blocks seen on the ebonite lid of the apparatus. These blocks are drilled to receive the plug shown, which should always be inserted when the condenser is out of use in order to dissipate any residual charge which might otherwise remain in it, thus leading to subsequent

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The ohm and volt, the respective units of electromotive force and resistance, have several different values, according to their origin. The history of the latter in detail would fill several pages with matter which is not of immediate interest here, and the writer has therefore appended the following table of comparisons by means of which the value of any particular unit can be deduced in terms of any other of the same kind.

The B.A., or “ British Association ohm, and the Ordinary” ohm are synonymous, as are also the

Standard ohm, the · International ? ohm, and the B.O.T., or “ Board of Trade” ohm : International Ohm equals 1.00235 Legal Obm

1.01358 B.A. Legal

0:99765 International

1:01120 B.A.
B.A.

0.98660 International
0:98892 Legal
0.9864. True

1.048 Siemens Unit True

1.0138 B.A.

Ohm Siemens Unit

0.9540 B.A. International Volt

Võlt

1 00235 Legal

1.01358 B.A. Legal

0:99765 International

1:01120 B.A. B.A.

0.98660 International

0.98892 Legal The reader will no doubt regard this multiplicity of values as somewhat confusing, and so, without doubt, it is, but, owing to the modifications which have been

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effected from time to time by certain authoritative bodies in our system of units, this confusion is unavoidable, some instruments and apparatus having been constructed before, and some after, each modification, so that the above table of comparisons, although somewhat crude, will prove an extremely useful one to the experimenter who has to deal with å multiplicity of apparatus dating back over a long series of years.

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Before departing from the subject of practical units and standards, there is one other elementary unit which ono seldom sees mentioned nowadays, and that is the mho or unit of electrical conductivity. It is an obvious fact that the conductivity or property of permitting the passage of an electric current, of any circuit, will depend upon the electrical resistance of that circuit; a matter of fact, it is the reciprocal of the resistance. The mho or unit of conductivity is the conductivity of a circuit having a resistance of one ohm, hence the word mho, suggested by Lord Kelvin, which, as will be noticed, is ohm spelt backwards.

Batteries.—For the majority of electrical tests it is obvious that we require a constant source of current ; this requirement is usually filled by a primary battery numbering as many cells of such capacity as will yield the maximum E.M.F. and current required. The type of cell most commonly employed in this country for testing purposes is the ordinary Leclanché element, which in its completed form is illustrated herewith. It is universally adopted for this purpose on account of its cheapness and constancy, within certain limits; it is also the least troublesome type of cell to maintain in working order, as, if properly set up in the first instance, it will work for months without requiring attention.

[graphic]

Ordinary Form Leclanché Element, by the India-Rubber, Gutta

Percha, and Telegraph Works Co., Ltd., Silvertown.

It consists of a square glass containing jar, with a circular neck; inside this is placed a porous pot containing a mixture of common gas coke reduced to granules, and powdered dioxide of manganese, in the centre of which is embedded a carbon plate. The latter has two holes drilled through it at its upper extremity, and is then immersed in molten paraffin wax for a depth of about an inch. A lead cap of the form shown in the illustration is then cast on, carrying a terminal screw and nut.

The object of dipping the upper extremity of the carbon plate in wax is to prevent “creeping" of the salanimoniac solution used as an electrolyte, which would otherwise tend to corrode the lead cap at its contact surfaces, and so sever the electrical continuity between cap and plate. The upper edge of the glass jar and porous pot respectively should be similarly treated with the same ulterior object, as the creeping trouble, especially in a slightly elevated temperature, will otherwise be a considerable drawback to the success of the cells. The top of the porous pot above the afore-mentioned mixture, which is packed to within about in. of its upper lip, is run in solid with marine glue, with the exception of two small vent holes, usually lined by small pieces of glass tubing, which serve for the ingress and egress of air and gas to the mixture in the porous chamber. In the outer glass jar is placed a rod of zinc, to the apex of which is soldered a length of copper wire for connecting purposes; the zinc rod requires to be well amalgamated or coated with a film of mercury before using, as otherwise what is known as “local action” is set up amongst the impurities in the zinc, and leads to the rapid destruction of the rod, which will

, if properly amalgamated, last for a considerable length of time, the consumption of zinc consequent on the working of the cell being very slight.

The electrolyte, which is poured into the outer jar, consists of a saturated solution of chloride of ammonium, or sal-ammoniac, as it is more generally known. "The solution is in the first instance poured into the jar to such a height as to completely fill it. The cell is then left for a few hours, when the level of the liquid will be found to have fallen an inch or more, due to its passage through the walls of the porous partition.

For light work, such as insulation resistance measurement and condenser work, where an infinitesimal current is taken from these cells, the saturated solution of salammoniac will be in itself a sufficient charge, but where larger currents are taken, as, for instance, in Wheatstone bridge work, etc., an inch or so of crystals should also be placed at the bottom of the outer jar in order to maintain saturation and thus keep the cell up to its work.

If worked too rapidly, the E.M.F. of this cell falls quickly owing to consequent polarisation, but if left to itself, will recoup within a comparatively short period. Its normal voltage is approximately 1.5, though when first set up it will be found to yield as high an E.M.F. as 1.6. The number of these cells required varies greatly with the test in which they are to be utilised, and may be anything between 1 and 500. If the number be small, they may be made up in box form, separated by

suitable partitions and provided with handles for mobility; where, on the other hand, the number is large, they should be mounted on insulated trays supported in suitable stands, placed in such a position that the cells are freely accessible on all sides for repairs, renewals, and recharging:

The Leclanché element is also made up in sets of a suitable number of cells for portable purposes. In this case, however, the cells are smaller, contained in square ebonite boxes sealed with marine glue, and the porous pot combination is replaced by an agglomerate block made up of the same materials, moulded under pressure and maintained in shape by an addition of gas tar or other suitable cohesive to the mixture.

Another form of cell which is largely used for testing purposes in America and on board ship is Warren de la Rue’s chloride of silver cell. Its popularity is probably due to its portability, the voltage and output of the cell being unaffected by the agitation consequent on moving from place to place. In its elementary form it is shown in Fig. 23.

B

Fig. 23.

B is a

It consists of a cylindrical glass containing jar, A, closed by a block or stopper, D, of paraffin wax. rod of chemically pure zinc, which constitutes the posi

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