R or www for (b) + for (a) C Q S ICi G Po Pi FIGURE 12.18. (a) Operational-amplifier electrometer circuit for charge measurement. (b) Op- erational-amplifier electrometer circuit for current measurement, for which the high-megohm resistor R replaces C.A condenser ion chamber is charged to P, = 300 V, then connected to the input of a high-gain charge-integrating electrometer such as that in Fig. 12.18a, giving a charge reading of Q, = 3.70 x 10 C. The procedure is repeated, this time giving the chamber an x-ray exposure of 10 R before reading out the charge at Q2 = 3.17 x 10 C. Repeating again with an exposure of 20 R, the charge is Q, = 2.65 x 10 C. Repeating again with 40 R, the charge is Q4 = 1.71 x 10 ' C. (a) Calculate the charge collected in the chamber per roentgen for each ex- posure. (b) What are the voltages corresponding to Q2, Q,, and Q,? (c) What can you conclude about the collecting potential being used