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7. Conductivity Measurement.-It frequently happens, in dealing with conductors of various kinds, that we require to discover what is known as the " percentage onductivity," i.e., the conductivity of the sample conductor under test, as compared with an exactly similar sample of absolutely pure copper, regarded as possessing a conductivity of 100. If my readers have met with many current specifications for electrical work they will in all probability have experienced the term "all copper used to have a conductivity of %." This means that a margin of so much per cent. is allowed for impurities and other causes affecting the conducting power of otherwise pure copper.

A method of determining the percentage conductivity of any given sample consists in cutting off a suitable length, such as 100 feet, for example, and carefully determining its resistance in ohms, by means of the Wheatstone bridge method previously described. It is then weighed in a delicate scale pan, the resultant weight being reduced to grains. The temperature also is carefully observed at the time of making the test; then the 12 × 22.61 percentage conductivity =

w kr

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The numerical values of k at various temperatures are given in tabular form in Kempe's "Handbook of Electrical Testing," and are reproduced herewith.

Co-efficients for correcting the observed resistance of pure copper wire at any temperature to 75° F., or at 75° F. to any temperature:

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Temp. F. Co-eff.

Table of multiplying co-efficients for reducing the observed resistance of ordinary copper wire at any

temperature to 60° Fahrenheit:

Temp. F. Co-eff.

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In some cases the diameter of the conductor in mils. is more readily obtainable than the weight of the sample tested. In such cases, the percentage conductivity 7 × 1065.6

where 1, k, and r represent the same quan

d2 kr tities as in the previous equation whilst d is the diameter of the sample in mils, or thousandths of an inch.

When dealing with conductors of fine gauge in which the correct diameter is somewhat difficult to determine, it is far better to resort to the weight method, by means of which, given a fairly sensitive balance, great accuracy can be attained.

For rapidly conducting a large series of conductivity tests on conductors of various sizes, Messrs. Nalder Brothers have designed the composite apparatus shown in the accompanying illustration. Figure 48 represents a working diagram of the apparatus, which consists in the main, of a series of ten carefully calibrated standards a, b, c, &c., of Iw, w, w, respectively, down to 1/512w.

These are connected at one end to a common 'bus bar A, and at the other to individual studs on a circular double contact switch B, which connects one end of them, respectively with a galvanometer terminal 1, and a variable resistance switch C, the movable arm of which is connected to one of the main terminals X. S is an adjustable slider, working on the common 'bus bar D, which is connected to No. 2 galvanometer terminal. and H are hinged steel knives enclosing the space of one metre between their respective edges. L is a metre scale, over which the slider S indicates. Galvanometer terminals 3 and 4 are connected to the 'bus bar A, and the knife F respectively. G is the high resistance galvanometer which, by means of the double switch J, can be connected across terminals 1 and 2, or 3 and 4, at will.

F

A differential galvanometer can also be used with this apparatus, the two windings being connected across 1, 2. and 3, 4 respectively. E is an accumulator, or cell, capable of giving a constant discharge rate of anything up to, say, 10 ampères.

The method of using this apparatus is as follows. A

(Copper Conductivity Apparatus, designed by Messrs. Nalder Bros., for rapid commercial work.)

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B

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S

FIG. 48.

length of the conductor whose conductivity it is required to measure is stretched as tightly as possible, without actually "killing" it, between the massive terminals X and Y. The hinged knives F and H are then brought down into contact with it without exercising sufficient pressure to nick or cut the conductor. The galvanometer switch J is placed in position on 3 and 4, and the slide S brought nearly up to the right hand knife H. The resistance switch C, which to start with was in the "off" position, is then manipulated until the largest readable deflection is obtained on the galvanometer scale. The switch J is then manipulated so as to make contact with 1 and 2, and thus bring the standard wires into circuit; the double contact switch B is then turned until that standard is included in the circuit which gives the next largest deflection to that already obtained on the galvanometer scale. We will call this deflection d. The switch J is now brought to 3 and 4, and the slider manipulated until d is again obtained on the galvanometer scale; then note the scale reading D of the slider S. Switch off the current at C, and cut out the metre length of conductor by depressing both knives F and H simultaneously. This length should then be weighed, and its weight W in grains noted, then the percentage conduc

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R. where R is the value in ohms of the

w-k

W

standard wire used, and k is a constant experimentally determined for the instrument.

When a differential galvanometer is used with this apparatus, the working is, of course, all effected to zero instead of to a given deflection d.

The apparatus is a very convenient one for rapid working, and saves considerable waste of material, as only one metre length of each sample is employed in the test. We will now leave the subject of resistance and conductivity measurement for a time, and proceed to a consideration of another very important matter, viz. :—

(8) The Determination of Electromotive Force. Like the matters already dealt with, there are several methods of arriving at the E.M.F. existing in a circuit, all of

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