Source of Illumination. Size and Distance. 151 



switchboard, as in this case the cross section of the wire has to be made to suit only the lamp, and the 

 loss of voltage (especially when installing direct current arc lamps) need not be taken into consideration; 

 accordingly a cross-sectional area of 2.5 sq. mm at 15 amps. 4 sq. mm at 20 amps, and 6 sq'. mm at 

 25 amps, is sufficient. If, however, this lead is only branched off in the lecture room, the common lead 

 must be of such thickness as to prevent any too large voltage fluctuations oceuring from the point of 

 introduction to the point where the cable branches off. The first method is more advantageous and 

 cheaper. 



It is possible, where means are limited to connect the projection lamp up to the lecture table, but 

 in this case the switchboard must not be used at the same time for other purposes such as is necessary 

 for the apparatus for lines of force. 



Nernst Light. The Xernst lamp method of illumination is specially convenient since it is seldom 

 necessary to replace the heater and the attention required is reduced to a minimum. The Xernst lamp 

 used for the projector necessitates a network voltage of at least 100; it is constructed for direct or alter- 

 nating current in voltages up to 260. Its candle-power at 110 volts is about 500, and at 220 volts. 

 1000 Hefner Candles (Fig. 50,923 on p. 171 shows a triple Xernst lamp constructed in the manner 

 suggested by Prof. Greil, and as inserted in the projection lantern). The burner must be heated with a 

 gas or spirit flame before use. The price list contains a self-igniting type also. 



Limelight and Thorium Light. These two well-tried types of glow light (of which the thorium 

 light is more efficient in its action) are simplest connected to the gas lead and the flame necessary for 

 heating the cylindrical or flat-shaped glower is generated by the aid of oxygen. The oxygen for this pur- 



>> pose can be made in an oxygen generator: it is much cheaper, however, to use oxygen bombs (steel 



flasks containing compressed oxygen), which are always ready for use and which are of the highest 

 value for projection purposes as well as for physical and chemical experiments. The flasks are sent 



& to the factory for re-filling. Fig. 50.95-4 (p. 173) shows an oxygen flask in which a capacity indicator 

 is inserted, so that the bombs can be recharged with oxygen at the proper time before the supply runs 



,S completely out. In addition to the indicator mentioned, however, it is advisable to provide a pressure- 

 reducing valve with special pressure gauge for the reduced pressure. Fig. 50.953. 



jg Two gas leads are used for conveying the illuminating gas and the oxygen respectively, in the 



case of the thorium and limelight burners. The igniting flame is put into contact with a part of the 



& thorium or lime slab which it renders incandescent. When the candle-power goes down, the glower 



is rotated thus rendering a fresh part of the same incandescent: after it is used up, the glower is replaced. 



The limelight burners with cylindrical glowers are arranged in the same manner. The thorium discs 



3 need to be renewed but seldom, 

 o 



If illuminating gas is not available, hydrogen can be employed instead, taken either from 

 a hydrogen generator or from a steel flask filled with compressed hydrogen. If a supply of illumi- 

 ^ nating <MS i> wanting, recourse may also be had to limelight burners for ether and oxygen 

 (Fig. 50,944. p. 172).' 



Fig. 50,955, p. 173, shows a limelight burner burning with gasoline and oxygen, contained, 

 together with all accessories in a trunk-shaped box. This lighting arrangement comprises: 1 small oxygen 

 flask, 1 pressure-reducing valve with pressure gauge) consumption indicator and key, 1 limelight burner 

 with gasoline receiver and the necessary flexible tubing, also lime disc or cylinder. This arrangement 

 is very convenient for travelling purposes. 



Spirit and Petrol Incandescent Light. The lamp for spirit incandescent light shown in Fig. 50,964, 

 p. 174, gives a very bright light only slightly inferior to limelight; and with its aid well-lighted images 

 up to 4 m length of side can be obtained. The lamp is provided with reflector and double incandescent 

 mantle. The initial pressure is produced with a small pressure pump which is given in, and the pressure 

 prevailing at any moment can be read off on a gauge. 



Size of Condenser and Distance between Projection Lantern and Screen. 



The size of the condenser is determined (1) by the size of the photo to be projected. (2) by the 

 distance at which the projection apparatus lias to be from the screen. The diapositives usually sold 

 have a free aperture of image of about 7x7 cm: for this si'/.e the 102 mm condenser is sufficient. 



For many cases, more especially the projection of apparatus, it is advisable to use a larger 

 condenser. 



