To measure motor RPM, the rubber-tipped shaft on a handheld tachometer must be held against the __________.

The correct answer is to hold the rubber-tipped shaft of the handheld tachometer against the end of the spinning motor shaft. This method is effective because the tachometer uses the contact with the rotating shaft to measure its speed in revolutions per minute (RPM). The rubber tip ensures that the connection is secure while preventing damage to the motor shaft and the tachometer itself.

When the device touches the spinning motor shaft, it can accurately gauge the speed of rotation. This is crucial for maintaining optimal motor performance and diagnosing any potential issues that may arise due to speed variations. The accuracy of the RPM reading is directly tied to proper contact with the active part of the motor, as opposed to other options that may not provide reliable measurements or could impede the operation of the motor.