A metering device has a magnet with ten poles which is driven to rotate by a gear connected to a source of rotation which is proportional to fluid flow, the passage of time, or other phenomenon. A reed switch is mounted so that rotation of the poles of the magnet, into and out of a position adjacent to the reed switch causes the reed switch to open and close as the magnet rotates. The opening and closing of the reed switch causes a step change in voltage applied to an electronic device which increments a counter. A second reed switch spaced from the rotating magnet sufficiently so that it remains normally closed can act as a sensitive device for detecting the presence of extraneous magnetic fields which might interfere with the operation of the first reed switch.