摘要:
A first and a second counter are provided. Each counter has at least one rotatable readout wheel. Drive means are provided for rotating the readout wheels. Internal planetary gearing is provided between the counters. The drive means causes the readout wheel of one counter to rotate at a predetermined race relative to and different from that of the readout wheel of the other counter. An externally driven revolvable mask may be disposed over the face of the numbers on the rotatable readout wheels of the counters. The mask when revolved in one direction exposes only the numbers of one counter and when revolved in counter direction exposes only the numbers on the other counter.