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RESISTANCE 49
material. Asbestos millboard, slate or marble is excellent.
Such insulation is a protection against slow current leaks should a ground occur between rheostat frame and resistance elements.
Ballast Resistance
(64) Ballast resistance — or its equivalent in the form of impedance when a transformer is used — is necessary to insure the steady operation of an electric arc because an arc between carbons has what is called a negative temperature characteristic. (65) The term "negative temperature characteristic" means that the voltage drop through the arc stream of an arc of uniform length decreases with increasing temperature.
(66) If the arc is connected directly to a constant voltage circuit, with no resistance or impedance in series therewith, the voltage drop in the arc stream decreases as the temperature rises, wmich of course has the direct effect of permitting more current to flow. This in turn creates still higher temperature, resulting in a further voltage drop in the arc stream. This condition quickly leads to an unstable arc, and stability in a projection arc is of paramount importance.
(67) The voltage drop across a ballast resistance — or its equivalent in transformer impedance — is equal to the product of the resistance and current flowing through it. (68) Therefore the voltage absorbed by the resistance increases with increasing current flow and decreases with decreasing current flow.
(69) When resistance or impedance is connected in series with an arc, it serves as a ballast or stabilizer of the current flow because the effect on voltage by changes in current flow are opposite to those in the arc stream. The drop in voltage in the arc stream which follows increase in temperature permits some increase in current. but this, in its turn, causes more of the line voltage to be absorbed in the ballast resistance, and thus reduces the voltage applied to the arc. Thus the increase of current is definitely limited or retarded, and consequently the action of the arc is stabilized.