Characteristics of Composite Carbon Resistors Used for Measuring Accelerator Terminal Voltages

A voltage divider board composed of 1152, 22 MΩ composite carbon resistors used to measure the terminal voltage of a 3 MeV Dynamitron was investigated. The board was calibrated in situ using the 9Be and D photoneutron threshold reactions and a magnetic analyzer. The present calibration of the divider board which was made after over 5 years of extensive accelerator usage agrees within 2% with the original factory calibration. The linear temperature coefficient of resistance was measured to be +(0.85±0.10)×10−3/°C in the range 25 to 35°C. The divider board resistance decreased linearly with voltage (−1.1×10−5%/V) up to 2 MeV which corresponds to 1750 V on each resistor. The results show that composite carbon resistors and the techniques described can be applied with confidence to the measurement of high voltage.