Megadiascope. 1217 



with a fine hole (' ._, millimeter) is placed in front of the megadiasc-ope. With the aid of a lens of about 

 7 ins. focus a sharp, magnified image of the fine aperture is obtained on the 9.84 to 13.12 ft. distant screen. 

 If the tuning fork apparatus is placed in the path of the ray of light issuing from the lens in 

 such manner that the ray is cast from the first mirror on to the second, and thence on to the screen, 

 the image appears on the screen. Since the path of the ray of light is lengthened on account of the 

 repeated reflection, the image must be focu>sed carefully again by sliding the lens. On the forks being 

 vibrated the Lissajous figure appears on the screen. 



Instructions for using Kohl's Megadiascope. 



The arc lamp must on no account be switched on unless the trough has previously been com- 

 pletely filled with water or else the glass panes of the trough will crack. 



The water is led into the trough on the side containing the cock. The latter is connected to 

 the water main by a length of hose: the outlet to the trough being connected by a second length of 

 hose to the sink or basin of tne water supply. 



The carbons should be introduced only when the arc lamp is switched off and not under current. 

 If it be desired to carry out this operation while the lamp is hot from use, the remaining carbon 

 is removed with the wood tongs and the key for loosening the clamping screws given with the apparatus. 



Before inserting the carbons, see that the carbon holders are drawn apart. 



^^ 



g A series resistance must be put in series with the network line. This can be adjusted so as 



to be invariable, i. e., for the normal current of the arc lamp (25, 30 or 50 amperes) or it can be ad- 

 justable, so as to regulate the current within certain limits. The latter arrangement is preferable. 



^ The current is regulated in the following manner: While the arc lamp and the coils of the rheostat 



'2 are cold the contact handle is placed on ''Weak", and after some minutes to the contact before the 

 last. If should then be observed whether the lamp (after the arc has inrecased in size by the burning 



away of the carbons) regulates properly. If this is not the case, the contact handle must be moved 

 r one contact back by way of experiment, and if this does not help it should then be moved two contacts 

 forward, i. e., on the last or "strong" contact. As a rule the lamp burns best when the handle is on the 

 last contact but one. 



It often happens that a molten peak forms on the thinner carbon. The lamp then emits a 

 hissing sound and does not give a bright light. After a short while this peak burns off of itself: it can 

 however be broken off by means of the wood tongs. It is advisable to separate the carbons before 



S switching on the current. This considerably reduces the chances of the formation of the peak mentioned, 

 if not eliminates it altogether. 



The arc lamp must be connected to the soiirce of supply in such 'manner that the thin carbon 

 is connected to the negative and the thick carbon to the positive pole of the network. Alongside the 

 switch are affixed small labels bearing the signs + (positive) and - - (negative), in accordance with 

 which the connections should be made. The polarity of the network is ascertained by the aid of pole 

 finding paper. A small strip of this paper is moistened with the finger and laid upon a table or a clean 

 wood board, the leads the polarity of which it is desired to determine being placed on the moistened 

 part of the paper about 3 cm. The pole finding paper is coloured red at the negative ( ) pole. 



The 220 volt rheostats have in addition to the terminals for 220 volts a third terminal for 

 110 volts so as to render it available for networks of the latter voltage. This third terminal is marked 

 "110 volts" while the terminal to be employed for 220 volts is marked "220 volts". One terminal is 

 the same for both voltages and is marked "110 and 220 volts". 



