VICTOREEN CONDENSER-METER 45 



of graphite, is earthed to the apparatus, and the central graphite elec- 

 trode of the " thimble chamber " is connected to a potential recording 

 device usually in the form of an electroscope or string electrometer. 



Fig. 1-17. The Victoreen condenser r-meter. A portable instrument for measur- 

 ing roentgens per minute. Its detachable chamber tube facilitates measurement 

 directly on the patient and simplifies phantom measurements. (By courtesy of the 

 Victoreen Instrument Company, Cleveland, Ohio.) 



Figure 1-17 shows one form of r-meter with a thimble chamber of the 

 condenser type, with its string electrometer calibrated in roentgen units. 

 It is a practical dosage instrument having a range from zero to 25 r. 



Victoreen Condenser-Meter 



In this type of instrument, shown in detail in Fig. 1-18, the chamber 

 at C is made of any substance having a effective low atomic number 

 (Bakelite). The thin chamber wall with its internally conducting 

 deposit of graphite, Fig. 1-19, constitutes one electrode of the ionization 

 chamber which is grounded to the case. The internal rod electrode C 

 of graphite is connected by a well-shielded conductor to the string elec- 

 trometer F. The string of the electrometer F, its connection, and the 

 graphite electrode are charged by means of a rotating amber wheel A. 

 The position of the deflected (charged) string is viewed through the 

 low-power microscope T. The image of the string is seen in the plane 



