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Group A Negatives
Group B Negatives
Amps.
Arc Voltage
Voltage
30
53
52
40
55
54
50
58
56
60
61
58
70
64
60
80
67
62
90
68
64
100
69
66
In the above table, the table of voltages given for the combinations in which the positives are matched up with small negatives of "Group B" it will be noticed that these voltages are somewhat less. It has been found in actual practice that a well shaped crater can be obtained at these lower voltages with negative carbons of diameters which are one-half that of the positive. The usual size negative is approximately two-thirds the diameter of the positive. These special small negatives are metal clad in order to give them carrying capacity and life since they are operated at current densities far beyond their possibilities were they not metal clad.
6. The electric arc when operating on Alternating Current is known to possess the peculiarity of varying in intensity to a marked degree with the peak and zero points of the alternations; being clearly visible to the eye on frequencies under 40 Cycles.
At commercial frequencies higher than 40 Cycles, this may be visibly demonstrated by moving under the arc a light colored pencil, or other object, against a dark background. When moved rapidly under direct sunlight or direct current arc, the pencil would naturally appear blurred, owing to the rapid change of position. Under the alternating current arc, the rapidly moving pencil would seem to be a number of pencils, each one clearly visible, with dark intermediate spaces. This is precisely the same effect as is obtained by moving the pencil in the light beam of a projector with the shutter revolving.
In reality, we are therefore dealing with two intermittent periods in the light source, when operating a film projector with an alternating current arc ; namely, the light interruption of the shutter and the rise and fall of the light intensity of the arc. The former depends for its frequency upon the speed of rotation of the shutter and the number of wings therein; while the latter depends upon the frequency of the current.
In determining the relation between current frequency and shutter movement, we might for example imagine a shutter with three 60degree openings and revolving at 40 revolutions per second.
This would give us 120 dark spots per second. An arc lamp on 60 cycle current would also have 120 dark spots per second. It is therefore evident that under these conditions, if the shutter opening occurred at the same instant at the zero point of the cycle, practically