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A NEW DYNAMIC LIGHTVALVE 389
covered when illuminated by the usual tungsten coiled-wire filament ; however, the moment of inertia is not small enough to allow locating all the resonances above the upper limits of 10,000 cps. Of course, it is advisable to place the fundamental frequencies as high as possible, if not altogether outside the range, thus avoiding complicated distortion-correcting devices in arriving at a flattened frequency curve. Simple damping answers the purpose. The main portion of the frequency curve, right from the beginning and under all circumstances, appears as a straight line. Considerations of this sort led to choosing for the damped loops a fundamental frequency of 7500 cps., and resulted in a frequency curve practically flat up to 10,000 cps. This was accomplished by a damping method to be described later, which in no way was influenced by temperature changes.
In arriving at the dimensions of the ribbons it is important to use as little power as possible for operating the oscillograph so as to permit building apparatus that would be light in weight and easily portable. However, since in the present case the power required is less than 0.5 watt, there is no advantage in reducing it still further, as the weight thus gained is insignificant compared with that of the entire apparatus. There is therefore no necessity of insisting upon a loop system having a maximum efficiency from an electromechanical standpoint; but it is quite essential to make use of as much power as possible for operating the apparatus in order to gain a large rotating angle. The greater the angle through which the vibrating mirror operates, the easier it is to effect a favorable and reliable construction of the comb-shaped shutter and the noiseless shutter. The power carried by the loop is not governed by considerations of stress and strength, but solely by the heating effect of the current. Excessive current impulses of short duration therefore are not important. The design of the ribbons is therefore determined solely by the increase of temperature resulting during steady full modulation. For that reason an effort should be made to dimension the ribbons so as to present large heat-radiating surfaces.
Further design features of the apparatus depend upon the following considerations: The thickness of the ribbons must be kept within such limits as to prevent their tendency to uncoil from causing irregularities in the deflection of the supporting spring. Furthermore, the width of the magnetic air-gap is determined by the width of the mirror, as the latter is located at the most effective point in the length of the ribbons — the center. It is not possible to make the central