176 



ELECTRICAL MEASUREMENTS 



In any a-c circuit containing both inductance and capacitance the actual 

 current flowing under a given voltage could be calculated from this 

 equation. Impedance defined in this manner becomes particularly im- 

 portant in communications where frequencies are high. The ideas are 

 used in biological instrumentation, of course, and should not be neglected 

 even in measuring electrical behavior of tissues. Living materials possess 

 electrical resistance, but there is also likely to be a measurable capaci- 

 tance within the living material, and another capacitance often exists in 

 the connection between the electronic instrument and the living ma- 

 terial. 



Vacuum Tuhes: Vacuum tubes are still used in a majority of instru- 

 ments for a variety of purposes. A very large number of different tubes is 

 available commercially, but in general they all depend on the same 

 principles. If a conductor with its loosely bound electrons is heated, 

 some of these electrons will escape from the surface. In a sense, they 

 are "boiled off." In an ordinary conductor, however, the electrons will 

 be immediately recaptured by the positive charges left on the surface of 

 the metal. At any instant, the hot surface will be surrounded by a "space 

 charge," a cloud of free electrons. If such a hot electrode is placed in 

 a vacuum, together with another "cold" positively charged electrode, 

 electrons leave the hot surface, travel across the intervening space, and 

 are captured by the positively charged electrode (called the plate or 

 anode). A "diode" operates in this way. (Fig. 13-2a shows such a tube.) 

 Obviously the tube must be connected into a proper external circuit so 

 that the hot cathode does not become positively charged by the loss of 

 electrons. Also obviously, the stream of electrons can move in only one 

 direction. Such a device could be used to rectify, that is, to change al- 

 ternating current into direct current. 



Anode 



Grid 



Cathode 



(b) 



Fig. 13-2. Diode (a) and triode (b) vacuum 

 tubes. 



