Conductors for electrical distribution; by Perrine F

Author:Perrine, F[rederic] A[uten] C[ombs], 1862- [from old catalog]
Language: eng
Format: epub
Publisher: New York, D. Van Nostrand company;
Published: 1903-03-25T05:00:00+00:00

results of his experiments the law of radiation as announced by Dulong and Petit in 1817, that :

h = c\\sx>^^\\syo^f — i)]

where h is equal to the quantity of heat lost per square centimeter of surface; c is equal to a constant depending upon the surface; 0 the temperature of the surrounding bodies; t the temperature elevation of the hot body above its surroundings. Experimentally for wires c was found to be equal to .05625» and assuming the average temperature of the air as 26° Cent., we have:

h = .5625[i.oo77»^(i.oo77'— i)] = .0687(1.0077' — i).

Applying this to the various experiments tried with small wires, large wires and flat strips, the loss of heat not represented by the radiation, but due to convection, was found to be approximately the same for all wires, and was represented by .00175 watts per linear centimeter per degree Centigrade temperature elevation. Taking now r as the specific resistance in ohms of the material used, we have for the energy developed per centimeter of length of a wire by I amperes:

4Pr(i +.003887^)

where T^=^t-\-Box the temperature attained by the wire, and as the energy developed is equal to the energy lost by radiation added to the energy lost by convection when a wire has attained a standard temperature, we have :

4i»r(i+.oo388r)

nd^

= .00175/ + ndm[.o6S7{i.oo77^ — i)]

where w is a coefficient of the surface of radiation equal to I for bright wires and 2 for blackened wires. Solving this, we

have:

/ 5707r^/[.o687 (i. oo77' ^ i)] + / /= 28.9^y V-+76^^ssT '

and for blackened wires, where m is equal to 2 :

40g^[.o687(i .0077' — .i)J + ^

r ,«^v /i i40^^ [-o687(i -oo7/ -

/= 28vy r+:cjo388r

CALCULATION OF CIRCUITS,

155

The calculation according to this formula may be much simplified by the use of a table giving the values of the heat radiated for definite temperature elevations, which is given by Kennelly as follows :

RADIATION TABLE IN WATTS PER SQUARE CENTIMETER FOR BRIGHT COPPER.

According to this table the carrying capacities of wires have been calculated for a temperature elevation of 19** Fahr., and are presented in the following table :

CARRYING CAPACITY OF WIRES SUSPENDED IN A ROOM (KENNELI.Y).

In experimenting with wires exposed to the air out of doors Kennelly found that it was exceedingly difficult to obtain accu-

rate results, as the slightest variation in the velocity of the wind would materially alter the convection of heat from the wire. In this case it was not possible, as before, to assume that the convection was constant for all sizes of wire, but that the total value of the convection could be obtained by adding to the constant term a term depending upon the diameter of the wire. The constant term being as before, .00175 watts per linear centimeter per degree Centigrade; the variable term being .oil d per degree, and for the total convection per linear centimeter per degree Centigrade elevation we have .00175 + .013 ^/ watts. Substituting this in the above equations we have for the carrying capacity of a bright copper wire suspended out of doors

^= 28.

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