Patent GB-519068-A (GB)
Title: Improvements in or relating to timing apparatus for clocks and watches
IPC: G04D7/12
Filed: 1938-08-11
Abstract: 519,068. Photo-electric testing of balance wheels. GIBBS, T. B. Aug. 11, 1938, No. 23753. Convention date, Aug. 16, 1937. [Classes 40 (iii) and 40 (v)] [Also in Group XVIII] Apparatus for testing the vibrating balance wheel 25 of a watch or clock consists of a stroboscopic device which includes a lamp 132 (133) and means for flashing this lamp under the control of light reflected from a moving surface of the wheel. Light from a source 8, Fig. 1, traverses lenses 4, 5, 6, mirror 10, and lenses 14 and 15 and is focussed at a point on the wheel rim from which it is reflected to a photoelectric cell 18. It is stated that when the balance is stationary (i.e., at rest or at either end of its swing) the output of cell 18 is constant. but when the wheel is moving there are current variations of complex nature. A high frequency component of these variations is used in the present instance to flash the neon lamps 132, 133. The amplifying circuits include a double triode amplifier valve 56, a valve 74, of which the left-hand triode functions as an amplifier and the righthand triode as a voltage regulator (i.e., limiting the maximum output), a potentiometer 82, a triode, double diode valve 100 which, with a transformer 104, acts as a rectifier, and a valve 110 adapted to energise a relay 111. Normally, with the balance wheel stationary, relay 111 is energized since valve 110 passes current in its plate circuit. Assuming that balance 25 beats five times per second, the arm 126 carrying the lamps 132, 133 is rotated five times per second by a synchronous motor 121 fed from an A.C. source of constant frequency. The balance is started in oscillation by the operator's finger. When the balance is moving, the flow of current in the plate circuit of valve 110 ceases, relay 111 is de-energized, armature 113 meets its upper contact and condenser 114 is charged through a rectifier R from a source P. When the balance is momentarily stationary at either end of its swing, the relay 111 is re-energized, armature 113 is moved to its lower contact and the condenser 114 is discharged through lamps 132, 133 in parallel to flash them, by way of brushes 138, 139 and slip rings 127, 128. The lamps show through an arcuate window 136 of an indicator panel 135 ; if the balance is oscillating too fast or too slow the point at which the visible rear lamp flashes will creep left or right respectively, Fig. 4. The balance is held in a holder so that it may be readily mounted and dismounted and the free end of the hairspring is placed initially by the operator in a peripheral groove of a roller 29, Fig. 1, where it is clamped by a knurled roller 28 mounted on a spring-pressd lever 27. As soon as the operator sees the error on the indicator 135, the hairspring 26 is adjusted left or right through the rollers 28, 29 and the test repeated until the balance keeps correct time. A handle 31, Fig. 3 (not shown) is then depressed to cause a knife 32, Fig. 1, to cut off the end of the hairspring and a die 33 to form a kink in the spring near the point of grip of the rollers. The balance and hairspring are then removed for assembly in a watch or clock. Limiting voltage output from valve 74. The left-hand triode of valve 74 acts as an amplifier and the fluctuations on the plate, due to the high frequency component of the voltage variations mentioned above, charge and discharge the condenser 78. The alternating voltages produced at the junction of condenser 78 and resistor 77 are applied to the right-hand grid of valve 74 and they vary in amplitude depending on the amplitude of the to and fro movements of the balance wheel. The function of this right-hand triode is to limit the effect of positive and negative half waves and maintain a substantially uniform output independent of the strength of the impressed voltages. For the negative half waves the effect is limited by appropriate adjustment of the grid bias of the right-hand triode by choosing a correct value for resistor 75 ; so that with signals stronger than desired the negative half waves bias the valve to cut-off. For the positive half waves the effect is limited by an appropriate value for resistor 76 (about 5 megohms) so that the potential fall across this resistor is so great that the grid potential can only rise a very little and the grid is never more than a fraction of a volt positive for positive half waves.