﻿334 Prof, R. Ball's Lecture Experiments to 



The convenience of this apparatus for every kind of mechanical 

 appliance in a lecture-room is wonderful. 



All bodies fall through the same height in the same time. 

 This was Galileo's experiment from the top of the tower of Pisa ; 

 it can be repeated on a small scale in the lecture-room in a stri- 

 king manner. 



A scaffolding is built up with the stool, beds, and bolts of the 

 system ; and thus at the height of 24 feet from the floor a pul- 

 ley is supported through which passes a rope. A piece of wood 

 the shape of an equilateral triangle, 9 inches each side, has two 

 electromagnets, each 3 inches long, attached to its base, the wire 

 being continuous round the tw r o magnets. The triangle is at- 

 tached by its vertex to the rope, and can therefore be easily 

 raised to 24 feet, or any less height, and lowered again at 

 pleasure. The two wires communicating with the electromag- 

 nets are sufficiently long to allow of the triangle and magnets 

 being hoisted up while the other ends of the wires are attached 

 to a battery of a few cells, a contact-breaker being introduced 

 into the circuit. 



The mode of experimenting will be easily understood. Sup- 

 pose it is desired to prove that a heavy iron ball and a light 

 one will fall through the same height in the same time. The 

 circuit being complete, the balls are attached one to each of the 

 electromagnets, care having been taken to interpose a piece of 

 paper between each of the balls and the corresponding magnet, 

 as this ensures their being disengaged simultaneously. The 

 piece of wood and its freight are then hoisted up 24 feet (or 

 less), and the two balls are in the same horizontal line, sup^ 

 ported, of course, merely by the attraction of the magnets. At 

 a signal the current is broken and the balls fall together; they 

 are disengaged simultaneously; and the line joining them is 

 easily seen to be horizontal throughout their entire descent, 

 though, of course, they are perfectly free from each other. The 

 iron balls used were 1 inch and \\ inch in diameter respectively. 



An iron ball (1J inch) and a cork ball (2^ inches) can be 

 likewise tried. A flat-headed nail driven into the cork affords 

 sufficient holding-ground for the magnet. It is a most unex- 

 pected result to find that when they reach the ground, or rather 

 the cushion placed to receive them, the cork ball is only a few 

 inches behind its weighty companion. That even this difference 

 is due to the resistance of the air is shown in the next experi- 

 ment, by lowering down the triangle, again affixing the cork 

 ball, and likewise the iron ball, w T ith a small parachute of card- 

 board attached to it. Raising up the triangle and again break- 

 ing the current, the cork is seen this time to reach the ground 

 before the iron. 



