189 



Tlie detectcir consists! of a pair of head telephones selected so as to have a 

 reasonable sensitivity when used under particular conditions met in these 

 measurements. The third branch is for the purpose of bringing the detector to 

 substantially the same potential as the metal shield on the inside of the case, 

 and this in turn is brought to substantially the same potential as the operator, 

 since the shield and the operator are electrically connected through the plate 

 on the outside of the cabinet opposite the hummer switch. 



The auxiliary cell is for the purpose of making the resistance Y+Q sub- 

 stantially the same in all respects as the resistance X. The mutual inductance 

 is to counteract any alternating electromotive force induced in the telephone 

 receivers or developed within the bridge as a result of inductive actions or 

 polarization at the electrodes of the measuring cell not compensated by a 

 corresponding- polarization in the auxiliary cell. 



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THE MICROPHONi; HUil.MEl; 



The micr<jphone Iiunmier is a device which operates on a direct current and 

 supplies a small alternating current in this case of a frequency of about 1,(X)0 

 cycles. A diagram of the connections is shown in diagram No. 3. 



Unless the carbon of the microphone button should become packed or burned 

 as the result of an excessive current, it should opei'ate satisfactorily for a 

 considerable time. An extra microphone button should be kept on board so that 

 in case of trouble a replacement can be made. With two ."0-watt llO-volt carbon 

 or two 40-watt llO-volt tungsten lamps, one in each compartment of the cabinet, 

 the voltage across the series resistance should be sufficient to give a satisfactory 

 operation of the hummer. The terminals of the hummer marked " Inter- 

 mediate " and " Zero " are used and normally furnish a test current so small as 

 to not materially affect the temperature of the sample:- of sea water being 

 measured, yet sufficiently large so that the bridge may 1»(- l>alanced to within 

 a single division of the slide wire or to within 0.01 in tlic ><alinity of the sea 



