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November, 1931
Motion Picture Projectionist
41
alent impedance can be calculated at any time by the following formula :
Zp = N2Z* where Zp = equivalent primary impedance Zs = secondary load impedance N = transformer ratio. Applying this equation to the above example where Zs = 10 ohms, and N = 4, then,
Zp= (4)2x10 = 16x10 = 160 ohms. The above equation can be represented in the following form when it is desired to determine the correct transformer ratio for a given power tube and a given speaker of known impedance as shown in Fig. 3:
N =
t
Zp
Zs
As an example assume a vacuum tube has an impedance of 1500 ohms. From the foregoing discussions it was found that for maximum undistorted power the load impedance should equal twice the tube plate impedance. Therefore, the load impedance of the tube or the equivalent primary impedance (Zp) of the transformer should be two times 1500 or 3000 ohms. Assume the voice coil impedance of the loudspeaker to be used is 30 ohms. Substituting,
N =
J Zp J3000J
= ¥100 = 10
Z* 30
The correct ratio of transformer to be selected is 10 to 1.
Principles of the Arc
(Continued from page 19)
The active element is composed of a mixture of oxides of iron, titanium and chromium. These ingredients are finely powdered and packed in an iron tube. This mixture produces an arc flame of a very white color which is very efficient for certain illumination purposes.
The mercury arc can be produced only in a highly exhausted glass or quartz tube. Its characteristic greenish color is caused by the peculiar spectrum of mercury vapor, which is strongly deficient in red rays.
Because of the great brilliancy of
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