PHOTOMETRY OF THE ELECTRIC ARC 129 



crater of the anode. The cathode gives a small amount, 

 but obstructs more than it gives out, while the gas between 

 the carbons absorbs as much as or perhaps more than it 

 gives out. The first person to give any statement con- 

 cerning the distribution of light from the arc which could 

 be put in simple form was Trotter. 1 His statement was 

 that the intensity of light in any direction was approxi- 

 mately proportional to the component of the area of the 

 crater which was perpendicular to and visible from that 

 direction. Thus above a in Fig. 39 little of the crater 

 would be visible, because of its concave shape, and what 

 is visible would have but a small component perpendicular 

 to the line drawn to a. As seen from b there would be 

 a large component perpendicular to the line drawn to &, 

 while at c it would be largely hid by the cathode. This 

 statement is, of course, only approximately correct, since 

 some light comes from the cathode and some from the 

 vapor, but Trotter showed that this was only a few per cent 

 of the total amount. 



The value of. an arc for lighting purposes depends to a 

 very great extent on the distribution. For example, the 

 distribution shown in Fig. 39 is undesirable, since almost 

 no light is thrown immediately below the lamp and 

 very little in a horizontal direction. The first defect 

 would make the lamp a poor one for indoor lighting, since 

 there would be a considerable dark region below the lamp. 

 However, such lamps are not often used for indoor work 

 and the more serious defect is the second one. Near the 

 lamp there will be light enough for street lighting in any 

 case, but as the distance increases the amount of light 

 rapidly diminishes, both because the distance is greater 



1 Lond. Elec., 28, 687; and 29, n; 1892. 



