APPENDIX V. The following is taken from the report of the Committee of the American Institute of Electrical Engineers. REPORT OF THE STANDARD WIRING TABLE COMMITTEE. MATTHIESSEN'S STANDARD OF RESISTANCE OF COPPER. Revised by the Committee on Units and Standards. The Council of the American Institute of Electrical Engineers, at its regular monthly meeting held December 3d, 1889, appointed a Committee "To formulate and submit for approval a Standard Wiring Table for lighting and power purposes." The Committee appointed consisted of the following members of the Institute:-Thomas P. Conant, Dr. Louis Duncan, Prof. Wm. E. Geyer, A. E. Kennelly, Geo. B. Prescott, Jr., E. Wilbur Rice, Jr., Prof. E. P. Roberts, Prof. Harris J. Ryan, William Stanley, Jr., Dr. Schuyler S. Wheeler, and Francis B. Crocker, Chairman. This action was taken by the Council with the object of overcoming or reducing the great confusion which now exists in regard to the standards and constants of electrical conductors. At the first meeting of the Committee, held January 10, 1890, it was decided to confine the work for the present to the three subjects of "Standard of Resistance," "Temperature Co-efficient" and "Safe Carrying Capacity of Copper," since these are of fundamental importance. The subject of "Matthiessen's Standard" alone is so confused and involved, and the discrepancies in regard to it are so great between the best authorities, as shown by the accompanying table No. 1, that the Committee has devoted its attention almost entirely to this subject up to the present time. TABLE No. I. Table of valuations for the Specific Resistance of Pure Copper at 0° C. in Legal Microhms, selected from various electrical papers and text-books. The Committee after careful consideration came to the conclusion that Matthiessen's "mile-standard" (one statute mile of copper wire inch in diameter having a resistance of 13.59 B. A. units at 15.5° C) is not the correct one, although very commonly used. Matthiessen himself did not place much confidence in this "mile-standard." The Committee, acting under instructions from a meeting of the Institute, held September, 1890, has based all standards and values in this report upon soft or annealed copper, since its properties are reasonably constant and reliable. It has purposely excluded from its recommendations all standards and values based upon hard copper, although several were given by Matthiessen, because the hardness of copper is merely relative, and the resistance of hard copper may vary between wide and uncertain limits depending upon the degree of hardness. As to the fact often brought up that copper may be found which shows one or two per cent. higher conductivity or less resistance than Matthiessen's standard, we are of the opinion that this is no real objection provided the value of the standard is definite and generally accepted. A standard which is not the highest attainable value may even be considered an advantage since the average commercial wires will approximate to it more closely. Although we believe the standard we recommend will answer the purpose temporarily and probably permanently, nevertheless we think that if a thoroughly correct and complete redetermination of the standard resistance of copper could be accomplished, it would be a benefit to electrical science and industry. Favorable offers in this direction have already been received by this Committee from Johns Hopkins University, Cornell University, and Columbia College, and it is likely that this redetermination may be undertaken. The following statement of the most important and reliable figures and facts given by Matthiessen will serve to show the derivation of the standard which we recommend. A hard-drawn copper wire 1 metre long weighing 1 gramme ("metre gramme") has a resistance of .1469 B. A. unit at the temperature of 0° centigrade.1 Matthiessen also gives the resistance of a hard-drawn copper wire 1 metre long and 1 millimetre in diameter ("metre millimetre") as .02104 B. A. unit at 0° C. This implies a specific gravity of 8.89 for the copper used by Matthiessen, but unfortunately he neglected to actually determine the specific gravity. Matthiessen's figures for relative conducting power are: From this the resistance of Matthiessen's hard copper is found to be 1.0226 times that of soft copper, therefore, the resistance of a soft copper wire 1 metre long, weighing 1 gramme "metre gramme" is 0.14365 B. A. unit at 0° C., and this is the fundamental standard recommended by the Committee. From this standard, with the specific gravity of copper 8.89 assumed by Matthiessen, are derived the following sub-standards: A soft copper wire 1 metre long and 1 millimetre in diameter ("metre-millimetre") has a resistance of 0.02057 B. A. unit at 0° C. A cubic centimetre of soft copper has a resistance of 0.000001616 B. A. unit at 0° C. 1 Philosophical Magazine, May, 1865. 2 Philosophical Transactions, 1864, A soft copper wire one foot long, and one thousandth of an inch in diameter (mil-foot") has a resistance of 9.720 B. A. units at 0° C. Taking one B. A. unit as .9889 legal ohm or 0.9866 new standard ohm, any of the above values may be converted into these units. The conductivity of copper at temperatures other than 0° centigrade may be determined by using Matthiessen's formula C1 = C. (1.00387 t+.000009009 t 2) in which C, is the conductivity at the given temperature, C, is the conductivity at 0° and t is the given temperature in degrees centigrade. It should be carefully noted, however, that this formula refers to conductivity. Therefore in order to apply it to resistance it is necessary to take the reciprocal and this should not be done by merely changing signs, which is not mathematically correct although often given in that way. The correct modification of Matthiessen's formula when referred to resistance is difficult to express accurately for any considerable range of temperature without increasing the number of terms in the formula, which would be very objectionable for practical work. The Committee has therefore calculated a table of temperature coefficients within the range of ordinary requirements. This seems to be the best. plan to secure accuracy and convenience. |