430 OPTICAL PROJECTION 



steadiness compared well with that given even by the continuous 

 current. 



The rectifier consists essentially of a revolving commutator or 

 reverser, driven in synchronism with the current by means of a 

 motor, and which again converts the alternating current into one 

 pulsating and unidirectional. The instrument, however, requires a 

 good deal of adjustment, and for ordinary arc-lamp demonstration 

 is hardly practicable. 



A motor generator, or a continuous -current dynamo driven by an 

 alternating-current motor, is the only real way of solving the 

 problem up to the present, and in large halls, where first-rate results 

 are really required, such a plant had better be laid down. Such a 

 machine is, however, costly, for even for a 10-ampere arc lamp a 

 generator costing 407. to 501. will be required, and for general 

 exhibition work quite good enough results can be obtained with 

 the alternating current alone. 



Should such a motor generator be purchased, however, it must 

 not be forgotten that with the continuous current, as with alternating, 

 80 volts or so is required, and a resistance in the circuit, in order to 

 avoid continual adjustment of the feeding mechanism of the arc. 

 This point is absolutely essential to a comfortable working of the 

 hand-fed arc lamp, and is too often overlooked. 



Automatic Arc lamps suitable for any ordinary sized lantern 

 are also obtainable ; the best is probably Borland's * Scissors ' 

 pattern, and under favourable conditions and in skilled hands this 

 lamp gives very satisfactory results. 



As the feeding of these lamps depends on the diminution of 

 current consequent on the burning away of the carbons, and conse- 

 quent increase of resistance, one of the essential conditions of smooth 

 working is that a slight variation in the length of arc shall produce 

 as much difference in this resistance as possible. For this reason 

 automatic lamps, unlike the hand-fed type, usually work at their 

 best with a voltage as nearly as possible that required for the arc 

 itself, say 45 to 50 volts, and very little, if any, resistance in the 

 circuit. With anything over 100 volts at the mains the feeding is 

 apt to be ' jumpy.' As a rule, of course, there is no control over 

 the current supplied, and we have to put in a suitable resistance to 

 do the best we can, but where a dynamo is being put in for the 

 purpose, this distinction between hand-fed and automatic lamps 

 must not be lost sight of. Col. Holden has designed a semi- 



