Houston and Thomson.] ™v [Nov. 1, 



In some of our measurements the resistance of the arc was surprisingly 

 low, being in one instance .54 ohm., and in another .79 ohm. It was how- 

 ever in some instances as high as 3.18 ohms. 



It may be noted as an interesting fact that where the greatest current 

 was flowing, the resistance of the arc thereby produced was low. This is 

 undoubtedly due to higher temperature and increased vaporization from 

 the carbons. In this latter case also the greatest amount of light was pro- 

 duced. 



The amount of work appearing in the arc as measured by the number 

 of foot pounds equivalent thereto, is not necessarily an index of the 

 lighting power. In two instances of measurement, the amount of energy 

 thus appearing in the arc was equal, while the lighting powers were pro- 

 portionately as three to four. This apparent anomaly is explained by con- 

 sidaring the resistance of the arc, it being much less in the case in which 

 the greater light was produced. The heat in this case being evolved in less 

 space, the temperature of the carbons, and therefore their light-giving pow- 

 ers, was considerably increased. 



A few remarks on the economical production of light from electrical cur- 

 rent may not be out of place. The light emitted by an incandescent solid will 

 increase as its temperature is increased. In the voltaic arc the limit to 

 increase of temperature is in the too rapid vaporization of the carbon. 

 Before this point is reached, however, the temperature is such that the 

 light emitted is exceedingly intense. No reliable method of measuring 

 the temperature of the arc has as yet been found. 



A well known method of obtaining light from electrical currents is by 

 constructing a resistance of some material such as platinum having a high 

 fusing point and heated to incandescence by the passage of a current. 

 When platinum is employed the limit to its increase of temperature is the 

 fusing point of the platinum, which is unquestionably but a fraction of the 

 temperature required to vaporize carbon. Were the falling off in the 

 amount of light emitted merely proportional to the decrease in tempera- 

 ture, the method last described might be economical. Unfortunately how- 

 ever for this method, many facts show that the decrease in the light 

 emitted, is far greater than the decrease of the temperature. Most solids 

 may be heated to 1000° F., without practically emitting light. At 2000° 

 F., the light emitted is such that the body is said to be at a bright red. At 

 4000° F., the amount of light will have increased far more than twice, 

 probably as much as four times that emitted at 2000° F. It is reasonable to 

 suppose that with a further increase of temperature, the same ratio of in- 

 crease will be observed, the proportionate increase in luminous intensity far 

 exceeding the increase in temperature. 



It would therefore appear that the employment of a resistance of pla- 

 tinum or other similar substance, whose temperature of alteration of state 

 as compared with that of carbon is low, must be far less economical than 

 the employment of the arc itself, which as now produced has been esti- 

 mated as about two or three times less expensive than gas. 



