The correct answer is D. All of the above.
A motor meter is a device that measures the speed of a motor. It consists of three essential parts: an operating torque system, a braking device, and a revolution registering device.
The operating torque system provides the torque that drives the motor meter. It is typically a spring-loaded system that is engaged when the motor is turned on.
The braking device stops the motor meter from spinning when the motor is turned off. It is typically a friction brake that is engaged when the motor is turned off.
The revolution registering device counts the number of revolutions that the motor makes. It is typically a gear train that is connected to the motor shaft.
The motor meter is an important tool for measuring the speed of motors. It is used in a variety of applications, including industrial machinery, automotive engines, and electric motors.