392 Scientific Intelligence. 



Mag., Jan., 1900), and remarks upon the sentence, " What would 

 appear to be wanted is some escape from the destructive sim- 

 plicity of the general conclusions." "The simplest way of 

 arriving at this desired result is to deny the conclusion, and so, 

 in the beginning of the twentieth century, to lose sight of a 

 cloud which has obscured the brilliance of the molecular theory 

 of heat and light during the last quarter of the nineteenth 

 century." — Phil. Mag., July, 1901, pp. 1-40. j. t. 



12. The Resistance and Electromotive forces of the Electric 

 Arc. — W. Duddell has carried on a series of experiments having 

 as their object the measurement of the resistance of the electric 

 arc under various conditions, as also of the electromotive forces 

 present. The method adopted involved the use of a steady cur- 

 rent to which a testing current of high frequency (120,000 

 periods per second) was added. The measurements of impedance 

 and the power factor were made by the three voltmeter method, 

 employing (1) an alternator for high frequency ; (2) a new, very 

 delicate instrument, called a " thermo-galvanometer," to measure 

 the three voltages and (3) a standard resistance for comparison 

 of the impedance of the arc, having a time constant of only 

 2*7 X 10~ 7 second. 



Some of the results obtained are as follows. The true resistance 

 of an arc 3 mm long between 1 l mm solid carbons, through which a 

 9*91 ampere current was flowing, was 3*81 ohms. The whole dif- 

 ference of potential was 49*8 volts, of which 37*8 was accounted 

 for by the ohmic drop, the real E M.F. opposing the flow of cur- 

 rent being 12 volts. With cored carbons the resistance was found 

 to be 2-54 ohms and the back E.M.F. 16*9 volts. With increase 

 of the direct current the back E.M.F. first decreased and then 

 increased, the minimum value being 1T3 volts for 6 amperes. 

 With the direct current constant, increase of arc length increased 

 five resistance. Further the composition of the electrodes was 

 found to have great influence on the resistance and back E.M.F. 

 Thus with direct current and arc length constant, soaking the 

 solid carbons in potassium carbonate reduced the resistance 

 from 3-81 to 2*92 ohms and increased the back E.M.F. from 

 12 to 15*6 volts. It is suggested that perfectly pure carbons 

 would probably make the resistance so great that the mainten- 

 ance of the arc would be impossible, traces of impurities being 

 essential 



The author concludes that the arc resistance consists of three 

 parts ; thus, for the case above noted, of (1) the resistance at or 

 near the contact of the positive electrode and the vapor column 

 of 1*61 ohms ; (2) the resistance of the vapor column of 2*5 ohms ; 

 (3) the resistance between the latter and the negative carbon of 

 1-18 ohms. The back E.M.F. consists of two parts located at 

 or near the contact between the electrodes and the vapor column. 

 That at the positive electrode, about 17 volts, opposes the flow of 

 the direct current, while that at the negative electrode, about 6 

 volts, helps the flow of the direct current, i. e., is a forward E.M.F. 



