Page images
PDF
EPUB

7. Conductivity Measurement.--It frequently happens, in dealing with conductors of various kinds, that we require to discover what is known as the “percentage nonductivity,” 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

22 x 22:61 percentage conductivity

w kr
where l represents the length tested in feet.
» weight

grains
resistance in ohms.
k

temperature co-efficient. 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 750 F. to any temperature :: Temp. Temp. Temp.

Temp. n F. Co-eff. in 'F. Co eff. in °F. Co-eff. in 'F. Co.eff. 100 .9484 82:5 .9842 65 1:0214 47:5 10601 99.5

9494 82 .9853 64.5 1.0225 47 1.0612 99 9504 81:5 .9863 64 1 0236 46:5 1:0623 98.5 9514 81 .9874 63.5 1 0247 46 1:0634 98 9524

80-5

.9884 63 1.0258 45.5 1 0646 97.5

.9534 80 9895 62:5 1 0269 45 1.0657 97 .9544 79.5 9905 62 1.0280 44:5 1.0668 96.5 •9554 79 .9916 61.5 1.0290 44 1:0679 96 .9564 78.5 .9926 61 1.0301 43:5 1:0690 1:010 415 1.039 81.5 9560 68 9834 54:5. 1.012 41 1.041

[ocr errors]
[ocr errors]
[ocr errors]

60

Temp.

Temp. in °F. Co.eft. in °F. 95.5 .9575 78 95 .9585 77-5 94.5 .9595 77 94 .9605 76 5 93.5 .9615 76 93 .9626 75.5 92.5 .9636 75 92 .9646 74:5 915 .9656 74 91 .9666

73:5 90 5 .9677

73 90 .9687

72 5 895 .9697 72 89 9708 71.5 88.5 9718 71 88 .9728 70.5 87.5 .9738 70 87 .9749 69 5 86-5 .9759 69 86 .9769 695 85:5 9780 68 85 9790 67-5 84.5 9301 67 84 .9811 66:5 83:5 .9821 66 83 9832 655

Temp.
Co-eff. in F.
.9937 60-5
.9947
9958 59 5
.9968 59
.9979 58.5
9990 58
1.0000 57 5
1:0011 57
1:0021 56:5
1:0032 56
1.0042 55:5
1.0053 55
1.0064 54:5
1.0074 54
1.0085 53:5
1.0096 53
1:0106 52:5
1:0117 52
1.0128 51.5
1.0139 51
1.0149 50 5
10160 50
1.0171 49.5
1:0182
1.0193 485
1.0204 48

Temp. Co-eff. in F. Co-eff. 1:(312 43 1:0702 1.0323 42:5 1:0714 1.0334 42 1.0725 1.0345 411:0736 1.0356 41 1:0748 1.0367 405 1.0759 10378 40 1.0771 1.0389 39 5 1.0782 1.0400 39 1 0793 1 0411 38.5 1.0804 1:0422 38 1.0816 1:0433 375 1.0828 1.0444 37 1:0439 1.0455 365 1.0851 1.0466 36 1.0862 1.0478 35.5 1.0873 1.0489 35 1.0885 1:0500 34.5 1.0896 1.0511

34

1.0908 1.0522 33:5 1.0920 1.0533 33 1:0932 1.0544 325 1.0943 1 0556 32 1.0955 1 0567 31.5 1.0966 1.0578 31 1.0978 1.0589 30.5 1.0990

49

90

Table of multiplying co-efficients for reducing the observed resistance of ordinary copper wire at any temperature to 60° Fahrenheit:Temp. F.

Co-eff.
Temp. F.

Co-eff. Temp. F. Co-eff. Temp. F. Co-eff, 9392 76.5 .9661 63 9937 49 5 1.022 89.5 9402 76 .9671 62.5 .9948 49 1.023 89 9412 755 .9681 62 .9958 48:5 1.024 88 5 9421 75 .9691 61.5 9969 48 1.025 88 9431 745 9701 61 9979 47-5 1.026 87.5 9441 74 .9711 60.5 .9990 47 1.027 87 9451 73-5 9722 60 1.000 46:5 1.029 86.5 .9461 73 9732 59.5 1.001 46 1.030 86 .9471 725 .9742 59 1.002

45:5 1.031 85.5 .9481 72 9752 58.5 1.003 45 1 032 85 9491 71:5 .9762 58 1.004 44-5 1.033 84:5 .9501 71 .9772 57.5 1.005 44 1.034 84 9510 705 9783 57 1.006 43 5 1 035 835 9520 70 .9793 56.5 1.007 43 1.036 83 9530 695 .9803 56 1.008 42:5 1.037 825 9540 69 9814 55.5 1 009 42 1 038 82 9550 68.5 9824

55

[blocks in formation]

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 1 X 1065.6

where I, k, and r represent the same quand2 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 stand a, b, c, &c., of lw, tw, Iw, 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. adjustable slider, working on the common 'bus bar D, which is connected to No. 2 galvanometer terminal. F and I 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.

S is an

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

[merged small][ocr errors]
[ocr errors]

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

[graphic]
[merged small][merged small][ocr errors]

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 I 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

1

D tivity R where R is the value in ohms of the

k

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

« PreviousContinue »