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MANAGERS AND PROJECTIONISTS 423
presents a different aspect, because if it is made to give a certain given current flow when heated to working temperature there will be a tremendous rush of current when the arc is first struck and the grids are cold. Big cast iron rheostats should therefore have some means provided for cutting in extra resistance until the grids heat up.
The grid rheostat presents certain advantages, also certain disadvantages as set forth below:
Advantages. Disadvantages.
(a) Better able to withstand (a) It is more difficult to rehigh temperatures with place broken grids than out damage. to replace coils.
(b) Grids are less likely to (b) The grid rheostat is sag and become grounded much heavier than the to the casing than coils. coil rheostat of equal
capacity.
(c) Grids give longer serv (c) Grids can be broken by a ice than coils. heavy jar.
(d) They deteriorate very (d) Temperature coefficient slowly. low and less fixed, therefore the grid resistance is somewhat less reliable.
TEMPORARY REPAIR.— Should a coil or grid burn out it is quite possible to make a temporary repair as per Fig. 133, in which B is an asbestos covered No. 6 copper wire doubled, and C the broken coil. It is not necessary to describe the operation, because the illustration shows clearly how it is done. The reason for doubling the No. 6 copper is that it will be subjected to high temperature. Such a repair eliminates a single coil, but almost any rheostat used for projection purposes may be used temporarily with one of its coils cut out. It is, of course, understood that such a repair is only intended for strictly temporary use.
RHEOSTATS CONNECTIONS.— Rheostats may be connected into a projection circuit either singly, "in series" or "in multiple." The various series and multiple connections are perhaps one of the most puzzling things the novice has to contend with ; also a large percentage of the "operators," and even some projectionists do not understand the matter any too well, yet it is quite simple.
Fig. 134 is the diagrammatic representation of what is known as the series connection. In this connection we find two 2-ohm rheostats connected in such way that the resistance of both is opposed to the voltage as a single element.