34 LECTURES TO SCIENCE TEACHERS. 



mechanical energy or any other physical force is trans- 

 formed into heat, there is a definite proportion between 

 the mechanical force and the heat which is produced. 

 These experiments were conducted in the first place to a 

 great extent by Dr. Joule of Manchester. There are a 

 vast number of ways in which we can transform mechanical 

 energy, or chemical affinity, or electrical action into heat, 

 and Dr. Joule attempted to prove this in a variety of 

 different ways. The first apparatus which he employed in 

 the year 1840 is on the table. There is a powerful electro- 

 magnet which can be rendered highly magnetic by a current 

 of electricity. Between its two poles he caused this coil 

 of wire to rotate with great rapidity, and when the coil of 

 wire rotates between the poles of a magnet a current of 

 electricity is generated through the wire, and this generation 

 of the current of electricity causes heat in the wire, and 

 the heat produced in the wire is the exact result mechanically 

 of the work in turning it. But the presence of the magnetic 

 poles causes a resistance to the motion of this coil, and if 

 we can measure the amount of heat developed in this wire 

 we shall know the amount of resistance which has been 

 opposed to the motion of the wire. Joule filled this tube 

 with water, and after it had been rotated for some time, 

 and the resistance it overcame had been measured, he 

 measured the increase of temperature, and so arrived at a 

 rough approximation of the mechanical equivalent of heat. 

 He afterwards extended the experiment in a variety of 

 ways, and perfected the methods more completely until he 

 was able to get exactly the same result every time, when 

 he varied the conditions of the experiment. The most 

 perfect apparatus he was able to construct is exhibited 

 here. This is the celebrated water-agitator which was 

 employed by Joule, the first results of which were published 

 in 1843. There are a certain number of fans which are 

 capable of rotation. These fans or paddles rotate in such 

 a way that they cause the water to be extremely agitated, 

 owing to the fans having to pass through these openings 

 in the fixed radial planes. This apparatus was placed in a 

 copper vessel which was filled with water. Then on the 

 spindle which you see here two threads were twisted in 

 the same direction a great many times. These threads 

 were drawn away and passed over two pulleys in opposite 



