Collision Apparatus
   The apparatus at the left below, in the collection of Transylvania University, is by Benjamin Pike, Jr. of New York. On the right is a piece of unmarked apparatus in the Collection of Historical Apparatus at Harvard University. Both devices are used to study collisions between two or more ivory balls of the same mass. Analysis shows that if the collision between a stationary ball and one released from rest a known height to the side is elastic, the moving ball will stop dead and the target ball will reach the same height. Since all macroscopic collisions are somewhat inelastic, this height is never quite reached. The Harvard apparatus is missing several balls on the left-hand side. This arrangement is known as Newton's Cradle, and one ball swung in from the right will cause a single ball to recoil from the left.  This apparatus seems to be as popular today as it was in the eighteenth and nineteenth century, and is sometimes found on the desks of harried executives, who play with it to calm their nerves.

   The collision apparatus at the left is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. The seven spheres are made of ivory and are 4.0 cm in diameter. 

   At the right is an unmarked piece of apparatus at the University of Cincinnati. Except for the base, this looks much like the apparatus listed at $3.50 in the 1912 catalogue of the C. H. Stoelting Co. of Chicago.

   Some apparatus, like some professors, never seems to retire. This collision balls apparatus at the University of Texas in Austin is in regular use in the lecture halls. At some time the original wooden scale collapsed and has been replaced by one made of Plexiglas. 

   The apparatus probably dates from the early days of the University, which opened in 1890. The 1888 Queen catalogue notes that it has "a set of five imitation ivory balls of the highest attainable elasticity, two and a half inches in diameter, mounted with double cords on a strong mahogany frame, three and a half feet high, with graduated arc ... $30.00"

   Queen was a forward-looking company, and would have used plastic had it been available in 1900.

   The collision apparatus at the right is somewhat different from the examples above. Here, the two balls start in contact, and an exploding spring forces them apart. They have equal and opposite momenta, and the velocities can be inferred from the height to which the balls rise.

   The apparatus is unmarked, but strongly resembles that shown in the 1881 catalogue of E.S. Ritchie of Boston. The catalogue copy reads:
   "Illustration of Momentum, two ball pendulums, one of which contains a spring hammer, the other is of half the weight; two addition balls, one of one third and one forth proportionate weight; the spring is held back by a thread, which in the experiment is burned, an equal force is expended on each of the balls, which are then thrown distances proportionate to their masses, ... !2.00"

   The apparatus is at the physics department of Hobart and William Smith Colleges in Geneva, New York.

   This unmarked collision apparatus dates from the first quarter of the twentieth century. It is 83 cm tall, and is in the collection at Case Western Reserve University in Cleveland, Ohio.
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