CH. XI] MOVING PICTURE APPARATUS 403 



that the arc can be watched. This window should be of fairly 

 large size and directly opposite the crater of the arc. The glass 

 should be dark enough so that the eyes will not be tired by the too 

 great brightness and yet light enough so that the whole of the hot 

 carbon ends can be seen. 



Another convenient way to observe the arc is to bore a fine hole 

 in the side of the lamp-house away from the operator. This acts 

 like apinhole camera and an image of the arc is seen on the opposite 

 wall. A sharper image of the arc can be formed by using a long 

 focus lens over an opening in the wall of the lamp-house to focus an 

 image of the arc upon the wall. A spectacle lens of about 25 cm. 

 (10 in.) focus (4 diopters) will answer. The lens may be held by 

 any convenient clamp but must be adjusted for distance to get the 

 sharpest image, otherwise it is no improvement over the simple 

 pinhole. 



The lamp-house should be well ventilated as from ^ to 2 kilo- 

 watts of power, .7 to 3 horsepower, is converted into heat. While 

 the arc is going there must be some way for this heat to escape, 

 otherwise everything inside would melt. One of the principal 

 causes of condenser breakage is poor ventilation of the lamp-house. 

 The best ventilation is secured by having holes permitting air 

 circulation but no escape of light, at the top and near the bottom 

 of the lamp-house. The back of the lamp-house is sometimes 

 removed.' 



In many places the fire underwriters or the city, require that 

 these ventilating holes be covered with fine wire gauze, to prevent 

 sparks flying out. This requirement was invented by someone 

 who had the mistaken idea that an arc lamp was a fiery volcano, 

 vomiting out sparks and lava in all directions instead of a quiet, 

 well behaved sort of thing. It is true that a minute spark some- 

 times does fly up, but it is so light that it cannot do any damage. 

 Any small piece of the carbon tip which breaks off will fall to the 

 bottom of the lamp-house where a suitable tray should be pro- 

 vided to catch it. This tray is also useful to hold the short pieces of 

 hot carbon just taken out of the lamp when new carbons are put in. 



568. Condenser. The condenser is usually in a box which is 

 fastened to the lamp-house and moves with it. In front of the con- 



