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When using small diameter solid metal coated negatives on direct current we start at 52 volts for 30 amperes and increasing by 2 volts for each increase of 10 amperes, reaching 62 for the arc voltage at 100 amperes, a saving of 0.7 kw. or 10 percent, in arc wattage, than in case where the old style large diameter cored negatives are used, starting at ^^ arc volts for 30 amperes direct current, and increasing voltage and current in same proportion as recommended in former case.
The arc steadiness and crater formation is materially improved as is shown in the photo (Fig. 4) of equivalent combination in the new and old systems.
In the past when using cored negative carbons the basis for choice of the negative was a ratio of i for the negative diameter, to 1.65 for the positive diameter, or a cross-sectional ratio of 1.2.
Heating Limit— S' mdling Limit — LA"
i' A.C. Trim.
Fig. 4
Fig. 5
Under the table of Carbon Combinations for direct current projection, the new developed solid small diameter metal coated negative calls for a cross-sectional ratio of 1.4, the negative having ^ area of the positive.
What determines the size of a carbon for given service is the ability to stand up under it but the limiting factor differs in A. C. and in D. C.
On direct current the limiting factor is the crater. Since the temperature of a carbon arc is constant just as is the temperature of boiling water — be there a teaspoonful or a barrel full — so, by putting into the carbon more current, we merely increase the number of the hot, light-giving areas until finally the tip of the carbon or crater can no longer accommodate an increase and then no further increase of light is possible for that carbon. The body of the carbon is as yet unaffected by the current but the crater can no longer take care of further increases. This is the limiting factor and so we take the next larger sizes.
On alternating current the crater is but half the size of the crater formation on direct current, owing to the fact that the energy is divided equally between the upper and lower carbon; therefore, we can go still higher in current density on A. C. without reaching a crater limit but we now find that the carbon bodv cannot carrv an unlimited
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