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RESISTANCE
41
Figure 10
insulated from the frame essentially as shown in Fig. 10.
A is the rheostat frame, B is the holding bolt, CC are coil ends and the shaded portions are mica insulators.
In reassembling coils or grids be very sure this insulation is complete so that there is no possible electrical contact between the frame or the casing and the resistance elements.
There are many forms of rheostat, but they all do exactly the same thing in various ways. (39) Since each inch of such a coil or grid of given diameter or cross section offers a certain amount of resistance to current flow, which requires voltage consumption (break-down) to force the current through, naturally the longer the path the current must travel through coils or grids the greater will be the total resistance, and consequently the greater the amount of voltage consumed.
Examining the grid, Fig. 9, it is seen that it is equivalent to a long cast iron wire. The coil shown in the same figure will be stretched somewhat when mounted, so that its spirals will be separated.
The resistance per unit length of grid or coil is known exactly; also known is the amount of resistance which will meet the passage of any definite number of amperes under an initial pressure. Therefore it requires only a very simple calculation to determine the coil or grid length required to pass any desired number of amperes taken from a 110, 220, or other voltage circuit.
(40) A rheostat will deliver an appreciably higher amperage when cold than after it has become hot. This is not a serious consideration in the small rheostats used in projection, but it is a serious objection in very large ones.
(41) The advantages and disadvantages of the grid