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An air circulating pump that works on the principle of acoustic wind is described. The pump will move up to 700 l/min of air when powered at 60 cps by a 4-in. loudspeaker. The speaker diaphragm is the only moving part and could be protected by inert material when it is desired to seal in and circulate corrosive gases. Alternating current pickup from the speaker is so small as to be negligible in most instances.

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An apparatus for measuring resistivity and Hall coefficient of semiconductors is described. Using a new type of measuring circuit, it combines the best features of both ac and dc apparatus. Thus, on the one hand, errors due to thermal emf's and the Ettingshausen effect are eliminated while, on the other hand, the one necessary balance is still made on a simple dc potentiometer with a dc galvanometer as null indicator. Contact and lead resistances do not affect the readings. Results of resistance and Hall coefficient measurements on a germanium specimen are given as an indication of performance.

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