ELECTRICAL MEASUREMENT BY ALTERNATING CURRENTS 313 



these methods will take a prominent place in electrical measurement, 

 especially where fluid resistances, inductances and capacities are to be 

 measured. They also seem to me to settle the question as to standard 

 inductances or capacities, as inductances have a real constant which can 

 now be compared to 1 in 10,000, at least. 



The new method of measuring liquid resistances with alternating 

 currents allows a tube of quite pure water a meter long and 6 Tnm. 

 diameter having a resistance of 10,000,000 ohms to be determined to 1 

 in 1000 or even 1 in 10,000. The current passing through the water 

 is very small, being at least 500 times less than that required when the 

 bridge is used in the ordinary way. Hence polarization scarcely enters 

 at all. 



It is to be noted that all the methods 15 to 24 can be modified by 

 passing the main current through one coil of the electrodynamometer 

 and the branch current through the other. The deflection will then be 

 zero for a more complicated relation than the ones given. If, however, 

 one adjustment is known and made, the method gives the other equa- 

 tion. 



Thus method 18 requires R t E" R'R II = Q. Hence, when this is 

 satisfied we must have the other condition alone to be satisfied. Also in 

 method 22, when we know the ratio of the self and mutual inductances 

 in the coil, the resistances can be adjusted to satisfy one equation while 

 the experiment will give the other and hence the capacity in terms of 

 the inductances. 



Again, pass a current whose phase can be varied through one coil of 

 the electrodynamometer, and the circuit to be tested through the other. 

 Vary the adjustments of resistances until the deflection is zero, how- 

 ever the phase of current through the first coil may be varied. 



The best methods to apply the first modification to are 15 A, 16 A 

 and B, 18, 20, 21, 22 and 24. In these, either a Wheatstone bridge can 

 be adjusted or the ratio of the self and mutual inductances in a given 

 coil can be assumed as known and the resistances adjusted thereby. 



The value of this addition is in the increased accuracy and sensitive- 

 ness of the method, an increase of more than one hundred fold being 

 assured. 



As a standard I recommend two or three coils laid together with their 

 inductances determined and not a condenser, even an air condenser. 



