| 摘要 |
A photoelectric barrier switch is mounted in a transparent part of the casing 5a of the measuring apparatus. A recess is formed in this part of the casing 5a between a light source 2 and a light detector 3 so that the light ray 4 passes through the wall of the casing twice. The light ray 4 can be interrupted by introducing a body into the recess 6 from outside the casing 5a. The effect of stray light on the light detector is eliminated by electronic means. In another embodiment, a push-button, which can be actuated in opposition to the force of a spring, is introduced into the recess 6. The light ray 4 can be interrupted by the push-button. <IMAGE>
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