Electricity for public schools and colleges . he naked eye the arccannot be studied ; the extre-mities of the carbons and thearc itself presenting a dazzlingliquid brightness that, with the consequent irradiation effect, bafflesanalysis. Through dark glass we can, however, study the arc;and we then see that the source of the light lies in the extremitiesof the carbons, whence the arc springs across, and in the arc itselfThis latter presents a flame-like appearance. Mechanical transport of carbon.—When thus examined, orwhen a magnified image is thrown upon the screen by means of alantern, it is
Electricity for public schools and colleges . he naked eye the arccannot be studied ; the extre-mities of the carbons and thearc itself presenting a dazzlingliquid brightness that, with the consequent irradiation effect, bafflesanalysis. Through dark glass we can, however, study the arc;and we then see that the source of the light lies in the extremitiesof the carbons, whence the arc springs across, and in the arc itselfThis latter presents a flame-like appearance. Mechanical transport of carbon.—When thus examined, orwhen a magnified image is thrown upon the screen by means of alantern, it is seen that one carbon becomes hollow after some actual measurements of weight it seems that it is the + carbonthat wastes most, there being a continual transport of particlesfrom it across to the — carbon ; and that the hollow appearanceof this latter is due to the fact that this stream of transported par-ticles is deposited upon it in a crater-like manner. Of courseboth carbons are also consumed slowly when the arc is exposed to. 44^ ELECTRICITY CH. XXV. the action of the air. The transference from + to — carbon istherefore best shown when the arc is used in vacuo. This mechanical transport is obviated by the use of alternatecurrent machines. Temperature of the rt-rr.—According to some experiments otM. Rosetti, of Padua, the + carbon attains the higher tempera-ture. These temperatures vary when the current varies. Theexperimentalist in question believes that we can regard the —carbon to attain 2,500° C, and the + carbon 3,200 C, atleast. Definition of equivalent resislance.~\i under any given condi-tions of driving-^. J/.^ E, &c., we remove the arc and substitutesuch a resistance p that the current remains unaltered, then p iscalled the equivalent resistance of the arc. The reader will re-member {see § 16) that the arc offers a reverse as well as aresistance. Hence the expression equivalent resistance has not thesimple meaning of Chapter XIII. ; its
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