XVII. RADIOACTIVE TRACERS Oil 



amplify the minute currents produced by small amounts of radioactive 

 substances (23) and many instruments employing this method and a 

 variety of circuits are now on the market. 



The most important application of this method is in monitoring 

 instruments, discussed in Section G of this chapter. A few more ac- 

 curate instruments using this principle are available (82) and the cir- 

 cuits of others have been published. Instruments of this type gener- 

 ally give a reading on a meter which is directly a measure of the rate 

 at which the radiation is entering the chamber, hence they are (tailed 

 "rate-meters." Servicing the circuits in these instruments is not ii 

 job for an amateur, but most of the manufacturers will make neces- 

 sary repairs on instruments and the newer models are becoming more 

 reliable. 



2. Electroscopes 



Though the older metallic leaf electroscopes can still find some 

 application, they will be found too insensitive for many research 

 problems. Their use has largely been supplanted by the quartz 

 hber type of which the Lauritsen electroscope (25) is the most widely' 

 known example (83). This is an extremely useful instrument, quite 

 moderately priced, relatively rugged, and very easy to use. Since 

 it is felt that this is the best all-round instrument for a limited re- 

 search budget, it will be discussed at considerable length. 



The principle of an electroscope of either metal leaf or cjuartz 

 fiber type is that of an electrical condenser in which the case is one 

 conductor and the leaf or fiber system the other. An insulator of 

 sulfur, amber, or a plastic of high resistivity supports the moving sys- 

 tem within the case. The quartz fiber of the Lauritsen is made con- 

 ducting by a sputtered metal coat and is connected to and (when dis- 

 charged) is parallel to a metal wire. An unstable condition is pro- 

 duced in the fiber system and when it is charged the fiber will be bent 

 away from the wire; as the charge is neutralized by the collection of 

 ions, the fiber regains its normal position (see Fig. 3). 



Certain other features of the Lauritsen are important. There is a 

 T on the end of the fiber, which is brought into focus on the scale of 

 the optical system, and the rate at which this T crosses the scale is 

 timed by a stop watch and recorded. A charging button outside 

 the case makes contact between the fiber system and the external 

 charging circuit that provides the necessary voltage. Though elcc- 



