QuickGuide to the Behr Free Fall Apparatus



The Behr Free Fall apparatus is a spark-timer device which is used to measure the acceleration undergone by an object in free fall. An electromagnet holds a shuttle suspended in air. When the electromagnet is turned off, the shuttle drops to a padded bin below. As it falls, it is at all times between a conducting rod and a grounded wire. In the image below, the conducting rod is on the right and the grounded wire is on the left. The conducting rod is connect to a 10 kV (that's a pretty high voltage -- do not touch!) power supply which turns quickly on and off every 60th of a second.

This 10 kV pulse is not great enough to spark from the conducting rod to the grounded wire. However, there is a conducting ring near the top of the shuttle, and the 10 kV pulse will jump from the conducting rod, to the conducting shuttle ring, to the grounded wire. This means that the shuttle creates a trail of sparks all the way down as it falls between the conducting rod and the grounded wire.

In order to see evidence of the sparks, a strip of carbon paper is placed between the falling shuttle and the grounded wire (see image). As the shuttle falls, the sparks traveling from its conducting ring to the grounded wire pass through the carbon paper. After the experiment, when the power supply is turned off and the strip of carbon paper is removed and examined, one can see a series of small dots indicating a spark burn. Since the time between each dot is known (one 60th of a second), the positions of the dots can be determined in order to discover something about the motion of a freely falling object.


One of the difficulties of setting up the Behr Free Fall apparatus is getting it perfectly vertical. If it is leaning slightly, the shuttle will either touch part of the apparatus as it falls, impeding its free fall motion, or else it will become far enough removed from the conducting rod and grounded wire that sparks will no longer be generated.

The apparatus can be made vertical using a plumb bob and the three adjustable feet of the apparatus. Suspend on end of the plumb bob from the electromagnet such that the other end hangs along the path taken by the shuttle. Adjust the feet until the plumb bob is exactly in between the conducting rod and the grounded wire.

behr free fall

Behr Free Fall Apparatus


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