NUMBER OF IMPEDANCES OF AN w TERMINAL NETWORK 303 



The classification under which the enumeration is conducted is 

 illustrated by Fig. 2, which shows typical positions of source, voltmeter 

 and ammeter for measuring impedances of three classes. In the first 

 of these, the ammeter reads the source current, the voltmeter source 

 voltage (across some pair of the network terminals) and the class is 

 that of driving-point impedances, D„. In the second class, that of 

 transfer impedances Tn, there are two types of connection: in the 

 first the ammeter reads the source current, the voltmeter a non-source 

 voltage, the voltage-current ratios being open-circuit transfer im- 

 pedances; in the second the voltmeter reads the source voltage and 



DRIVING POINT 

 IMPEDANCES (Dn) 



'-<I>-1 



OPEN CIRCUIT SHORT CIRCUIT 



TRANSFER IMPEDANCES (in) 



UA 



GENERALIZED TRANFER IMPEDANCES (Un) 



Fig. 2 — Arrangemeftt of apparatus for measuring impedances of three classes. 



the ammeter a non-source current, the voltage-current ratios being 

 short-circuit transfer impedances. It will be noted that the two 

 connections differ only in that the ammeter and voltmeter are inter- 

 changed. The third class is that of generalized transfer impedances 

 Un, in which both voltmeter and ammeter are across non-source 

 terminals. 



The last class, of course, might be supposed to include the two 

 preceding ones but the separation proves convenient not only for 

 numerical work, as will appear, but also for keeping distinct both 

 well-recognized and formally different classes. 



