Beakman's Motor Speed Measurements

Please note that the measurement technique we use is not perfect. Thus, please take care in doing it and, should you find that your speeds are in excess of 60Hz, you will have
to get them confirmed by both a TA and an instructor (Braunstein, Michael or Connor).

1. Connect one channel of the 'scope across the battery. When the coil is not making contact with the battery, you should observe the battery voltage of a little more than 1.5 volts.

2. Set up the 'scope to measure a speed in the range you think your coil will turn (usually 10-100Hz).  Most first-try motors turn nearer 10Hz than 100Hz. At 100Hz, you can hear the motor hum. Use the frequency measuring feature of the 'scope.

3. Start the motor. When the coil is connected, the voltage should drop because the impedance of the coil is much smaller than the impedance of the battery (part of your task is to confirm this). You should observe a series of square pulses and the coil connects 
periodically to the battery, once each revolution. Set the time cursors to the beginning or
end of two consecutive pulses to get the frequency. For better accuracy, set the cursors to the beginnings of every 10th pulse and then multiply the frequency obtained by 10. This must be done for any really fast speeds.  Note that it is possible for the coil to connect to the battery more than once per cycle if it is bouncing too much. Make sure that the paperclip cradle used to hold the coil does not close around the coil axis to avoid this problem. Also, the use of a non-conducting spring (possibly made with an insulated wire) will keep the coil from bouncing. This will make the coil go faster and  make the measurement more accurate.

-kc