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168 MOTION PICTURE HANDBOOK
in Fig. 84. There are eleven of these, each delivering a different number of amperes, beginning at the lowest and gradually increasing, in four ampere divisions, according to the table below:
Contact
Volts.
r ■ ist
2d
3d
4th
5th
6th
;th
8th
9th
10th
nth
100
V
31
35
39
43
47
5i
55
59
63
67
105
31
35
39
43
47
51
55
59
63
67
7i
no
35
39
43
47
51
55
59
63
67
7i
75
115
39
43
47
51
55
59
63
67
71
75
79
120
43
47
51
55
59
63
67
71
75
79
83
The Preddey Economizer is 21 inches in length by 6 inches high by 6 inches wide. Its weight is 85 pounds.
To operate a dissolving stereopticon, the lamps must be connected in series, as illustrated on page — , and the economizer, must then be regulated to deliver a greater number of amperes than would be necessary if only one lamp were in use. The principle of operation is as follows: The A. C. projection arc, while its voltage varies somewhat, averages perhaps a little less than 35 volts when operating at its best — let us say between 30 and 35 volts. It therefore follows that some device must be used to reduce the pressure from no, or wdiatever voltage the lines carry, to the arc voltage. This may be accomplished in several ways, one of which is by the use of the Preddey reactance, or choke coil. In this device the current is passed through a coil of very heavy copper wire (No. 4). This wire is wound around a laminated iron core, made up of sheet metal. When current passes through the wire coil it sets up, through the medium of the core, a series of electro-magnetic waves, commonly known as "magnetic kick," which tend to oppose the passage of the current, and in consequence the voltage falls to the required pressure, which same may be varied by connecting to different contacts, each one of which cuts out, or cuts in a certain number of additional turns of wire in the coil. The makers of the Preddey Economizer recommend that ^ cored carbons be used with their apparatus, and that they be so set that the top carbon will be in line with and directly above the lower carbon, using hard carbons where the device is delivering above 55 amperes, and soft carbons below this point.
To connect the economizer, fasten it to the wall in a vertical position. It should be placed within convenient reach of the operator and with the name plate on top. Do not fasten to an iron-clad room without first attaching blocks of wood, covered with asbestos. Never fasten the economizer directly to the iron. Connect the regulating contact with the contact nearest