150 



Projection Apparatus and Accessories. 



the screen some considerable distance away from the lantern, and especially spectrum experiments: 

 it being possible to do this work only by employing an arc lamp or a Nernst lamp. 



The arc lamps are either supplied for regulating automatically or by hand. In the auto regu- 

 lating type the distance apart of the carbons while burning is automatically adjusted by an electro- 

 magnetic regulator, rendering these lamps very convenient in use. The lanterns with hand-regulated 

 arc lamp only need to have the carbons regulated by hand very seldom; and they can be recommended 

 for this reason, viz. because they can be burned with currents of varying strength, and the maximum 

 brilliancy can be attained at the desired moment by regulating. The hand-regulated lamps can be 

 burned either on direct or alternating current. 



A current at a pressure of at least 65 volts must be available from the network, since the contact 

 resistance of the lamps has to be in series. The voltage supplied by electricity works is in most cases 

 higher than this, being usually 110 or 220 volts. In such cases a series resistance suitable for the pur- 

 pose must be employed. For rectifying 65 volts alternating into direct current of the same volliiiiv 

 a Cooper-Hewitt rectifier is very well adapted (see No. 50,882). 



The most satisfactory kind of current to employ for the projector (as for experimental pur- 

 poses) is Direct Current, since when using D. C. the crater of the positive carbon renders good light 

 distribution possible, as it is possible to turn this crater towards the condenser by sloping one or 

 both carbons without the tapered negative carbon casting a shadow. Complete information as to 

 adjusting the carbons is given later. 



The lamp is connected to the network by a double flexible lead which is connected to the lamp 

 terminals, the free end being provided with a plug contact. To establish the connection the latter is 

 inserted into a plug box placed at a convenient height on the wall or suspended from the ceiling. The 

 series resistance just mentioned (which must be adapted to the network voltage and the lam]) current) 

 and also a 2-pole fuse must, when direct current is being used, be inserted in the lead coming from the 

 network to the plug box. To render the plug box devoid of current when not in use, it is prescribed 

 that a double pole switch be inserted in the circuit before the plug box. These pieces of apparatus 

 (fuse, switch and plug box) are mounted together on a small marble slab (No. 50,878, p. 169; see 

 also Fig. 50, 728 A). The switchboard is arranged for connecting up the lamp from the wall. When 

 connection is made from the ceiling, the plug box is omitted from the switchboard (see No. 50,879, 

 p. 169) and is given in separately, being of a special type. When ordering, kindly state the length of 

 lead desired so that this may be supplied ready connected up. 



The Series Eesistance is used for reducing the network voltage (which is usually either 110, 

 120 or 220 volts) down to 40 47 volts, i. e., the voltage necessary for working the arc lamp, by dissi- 

 pating the excess voltage. It serves at the same time as a 

 steadying resistance and ensures the lamp burning steadily ; it is 

 not recommended that the lamp be connected to, say, 50 volts, 

 without any series resistance at all. The resistance is adjusted 

 once and for all to that current and voltage on which the lamp 

 has to burn, and for which it is regulated. 



In connecting up a D. C. lamp care should be taken 

 that the upper carbon is joined up to the positive pole of the 

 lead, and that the plug is inserted with due regard to the -j~ 

 and - - signs to be found on it. 



In the case of Alternating Current Arc Lamps used in 

 conjunction with alternating or three phase stations, it is ad- 

 visable to employ a Static Transformer in lieu of the series resis- 

 tance, this transformer stepping the network voltage down to 

 that which has to be employed, which is about 30 volts. A con- 

 siderable saving in working costs is effected by this arrange- 

 ment. The A. C. lamps are only used arranged vertically ; both 

 carbons used are of the cored type, and both are of the same 

 thickness, or the lower is slightly thicker than the upper. 



Special care should be paid to the correct arrange- 

 ment of the electric leads. For the D. C. lamp, 15, 20 or 

 25 amperes current should be reckoned, while the 20 or 25 ampere 

 alternating current arc lamp at 120 volts network voltage, and 

 employing a transformer, takes 7 8 amps, from the network. 

 It is advisable to branch the arc lamp lead from a main lead in- 

 dependently of the lead for lighting and for the experimental 







50 728 A. 



Switchboard for the electric projection lamp 

 (direct current). 



n. L'L'.-,. 



