# ISA Interchange

Welcome to the official blog of the International Society of Automation (ISA).

# AutoQuiz: How to Measure the Resistance in a Circuit

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AutoQuiz is edited by Joel Don, ISA's social media community manager.

This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.

### A circuit has a 100 Ω resistor, a 50 Ω resistor, and a 200 Ω resistor all in parallel with each other. What is the total resistance of the circuit?

a) 28.6 Ω
b) 35.0 Ω
c) 175 Ω
d) 0.28 M Ω
e) none of the above

The correct answer is A, 28.6 Ω. To find the total resistance for resistors in parallel in a circuit, use the equation:

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition