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It is evident that current flicker is an inherent objection to alternating current arc projection. There are other faults better known, such as the bluish color of the light, the poor concentration of light at the light source focus of condensor, and the noisy hum of the arc.
Without entering into details of these other objections, which are apart from the subject matter of this paper, the mention of them seems permissible, as it completes the evidence against alternating current projection, and proves its use poor practice.
Since the direct current arc possesses none of these objections and since a large variety of thoroughly satisfactory and practical apparatus is on the market for converting alternating current to direct current, there seems to be no excuse for faulty projection, resulting from the use of alternating current.
For alternating current, cored carbons in both upper and lower are always used and these are the same size. This is essential, of course, since the rate of consumption is practically the same for both the upper and lower carbon. On direct current some operators use cored carbons, both upper and lower, and also use the same size carbon for the positive as well as the negative.
While it is sometimes desirable to use cored carbons, both positive and negative, it is the wrong principle to use the same size — the negative carbon should always be smaller than the positive.
For all around service the most advisable ratios will be such that the cross section of the positive carbon would be twice that of the negative carbon, but in actual practice we find that a negative carbon would not have the carrying capacity for the current required for the positive, if the latter were operated at maximum current density. Therefore, the following combinations are a compromise which have been standardized for various current densities. These are as follows :
MAXIMUM CURRENT.
Alternating
Direct
Plain Negatives
Metal Coated
Size
Current
Current
Direct Current
Neg. Dir. Cur.
1"
100
85-100
3/4 Cored or sol.
3/8 sol. spec, coated
7/8"
90
65-85
3/4 " " "
3/8 " reg. "
3/4"
75
50-65
5/8 " " "
11/32 " "
5/8"
60
50
1/2 '• " "
5/16 " "
9/16"
50
40
1/2"
40
35
7/16"
25
25
3/8"
15
15
5/16"
10
10
ynam
8
8
6»"
6
6
In the table of direct current capacities, from the 5/8" size up, the next smallest sized solid carbon is used as the negative.
Cored negative carbons give steadiness at the cost of the candle power of the arc. Under certain conditions it is possible to operate carbons at somewhat higher current densities than here given and still obtain satisfactory service. It is, however, essential, in order to