JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VIII OCTOBER 19, 1918 No. 17 



RADIOTELEGRAPHY. — New method of using contact detectors 

 in radio measurements. L. W. Austin, U. S. Naval Radio 

 Laboratory. 



For many measurements in radiotelegraphy it is necessary 

 to use a radio frequency-current indicator of known resistance. 

 If the current to be measured is small it is generally customary to 

 use a thermoelement and galvanometer. The most sensitive 

 thermoelements are either of the vacuum type or the welded 

 tellurium type. The vacuum thermoelements can be obtained of 

 any desired resistance and are very sensitive but are slow in 

 action and frequently show a bad zero drift. In addition, the 

 deflection usually shows considerable divergence from the cur- 

 rent-square law. The tellurium platinum elements are quick act- 

 ing and follow the current-square deflection law with sufficient 

 accuracy for all practical purposes. They are however so fragile 

 and difficult to manufacture and transport that no manufacturer 

 has yet undertaken to supply them commercially. It is also 

 impossible to make the contact resistance much less than 10 

 ohms. It is to be noted that the resistance in both the vacuum 

 and tellurium types changes considerably with the amount of 

 current flowing. 



On account of the difficulties mentioned, the sensitive thermoele- 

 ments in our laboratory have been replaced, for the most part, 

 by a shunted contact detector circuit arranged as shown in fig- 

 ure 1. Here LC is anj^ oscillating circuit having inductance and 



569 



