JOURNAL . 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VIII FEBRUARY 19, 1918 No. 4 



PHYSICS. — An optical ammeter. Paul D. Foote, Bureau of 

 Standards. 



The high precision in the measurement of temperature possi- 

 ble by use of a property designed Holborn-Kurlbaum optical 

 pyrometer equipped with a suitable pyrometer lamp is well 

 recognized. If the lamps are operated below 1300° to 1500°C. 

 the calibration of the pyrometer will not change appreciably 

 after years of ordinary use. These two factors, accuracy of 

 photometric settings and constancy in calibration of the lamps, 

 permit the adaptation of this method of photometry to the 

 measurement of current. In the present note two forms of cur- 

 rent measuring instrument are described, one of which is strictly a 

 hot-wire ammeter with the hot wire at a temperature between 

 600° and 1500°C., and the other a device for adjusting a current, 

 by optical methods alone, to any preassigned value. The first 

 method should prove especially useful for the accurate measure- 

 ment of alternating current of high frequency. 



In order to show what precision may be expected in measure- 

 ment of current through the pyrometer lamp when the instrument 

 is sighted on a source of constant brightness we will make the 

 assumption that a photometric match of the tip of the lamp fila- 

 ment against the uniform background can be made with an ac- 

 curacy of 0.5 per cent. This assumption is reasonable for pre- 

 cision photometry of this type, but even if the accuracy were 

 only 5 per cent surprisingly accurate measurement of current 

 can be obtained, as will appear below. The current tempera- 

 ture relation for a particular lamp used, G. E. No. 8, has the 



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