The Gaertner apparatus at the left was originally at Denison University and is now in the Greenslade collection. The falling body is the glass plate at the bottom of the apparatus. As this falls down, guided by the two upright rods, a point on the vibrating tuning fork in the middle scratches a sinusoidal trace down the light coating on the glass. The tuning fork runs at a constant frequency, but the sinusoid is stretched out as the falling glass accelerates downward. This gives information on the position of the falling system as a function of time. 

   At the right is a similar pieces of apparatus from Glasgow University that is probably locally-built.