Lightning from thunderstorms is dangerous in particular circumstances where explosives are being handled. The presence of a thunderstorm is determined by measurement of the earth's electric field by utilising an alpha-particle generator. A rotating horizontal screen (39) modulates the signal resulting from the earth's electric field. This is sensed by electrodes (7, 9) coupled to a differential input amplifier (59), whose output is filtered (61) to eliminate mains frequency. The output is applied to a capacitance diode (63) whose output is transmitted by cable (64) to a comparator (65) where a reference value is applied from a potentiometer (67). The comparator output is applied to alarm (71) and tert (73) circuits both of which receive signals from a control and sequencing circuit (75). This circuit activates a circuit (81) to provide a test voltage.
展开▼
