Patent GB-609556-A (GB)
Title: Improvements in stroboscopes
IPC: H05B41/34
Filed: 1946-03-15
Abstract: 609,556. Stroboscopes. THOMAS, E. L., and SPERRY GYROSCOPE CO., Ltd. March 15, 1946, No. 8168. [Class 97 (i)] A stroboscope comprises manually-operable flash frequency setting means and calibrated means associated therewith to indicate the flash frequency set thereby, the flash frequency being manually changeable to a predetermined multiple of, and restorable to, the frequency indicated without disturbance of the calibrated means. As shown in Fig. 1, a relaxation oscillator is coupled with a discharge lamp 1 so that the pulses produced cause periodic flashes of light. The flash frequency is governed by the time constant of an R.C. circuit comprising an adjustable resistance 11 and condensers 10, 12 of equal capacity. In use, the value of resistance 11 is adjusted until the object illuminated appears stationary, switch 13 being open. This switch is then closed and the flash frequency of the lamp 1 thereby doubled; if the object still appears to be stationary than the flash frequency indicated by means associated with a handle for adjusting the resistance 11 is equal to or an odd submultiple of the frequency of vibration of the object. A vibratile reed 19 as of ferro magnetic material and mounted on a rigid base lies in the field of an electromagnet 17 which is energized through the pentode 2. This reed serves to calibrate the stroboscope and may be such that its natural frequency is harmonically related to one or more of the flash frequencies determined by the resistance 11. In the form shown in Fig. 4, flash frequency determining means 210 is provided with a spindle 22 to which a manually-operable knob 23 is secured and is such that equal rotations of the knob 23 cause equal changes in the flash frequency. Slidable on the spindle 22 between a collar 25 secured thereto and a friction disc 31 on the base of the instrument is a clutch member 24, which is normally urged into engagement with the collar 25 by spring forks 30 attached to a lever 29, but which is movable by an eccentric 126 into contact with the disc 31. To the clutch 24 is fixed a pin 33 coupled by a spring 35 to a pin 36 on the collar 25, which is recessed at 34 to provide a lost-motion connection between the collar and the clutch. In use, the knob 23 is rotated until the flash frequency of the stroboscope, indicated by a pointer 38 fixed to the clutch 24 and co-operating with a scale 37, is such that an object appears stationary. The pointer 38 is now held fast by movement of the eccentric 126 to make contact between the clutch 24 and the disc 31 and then knob 23 is rotated until the pin 33 contacts the other end of the recess 34, which is of such an extent as to allow the flash frequency to be doubled; a scale 40 attached to the collar 25 is provided and co-operates with the fixed pointer 38 to indicate the new flash frequency. Another embodiment is described wherein a light-tight casing encloses a shutter which is rotatable at various speeds by an electric motor with which is associated a calibrated scale and a steady light source. The shutter comprises two coaxial and relatively movable apertured discs, the lower disc having a number of opaque portions equal to half the number of apertures in the upper disc so that in one position alternate apertures in the upper disc are covered by the opaque portions in the lower disc, whereas in the other position, all the apertures in the upper disc register with the transparent portions in the lower disc. Hence, these two positions give flash frequencies, one of which is double the other.