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Joule’s Experiments on Mechanical Equivalent of Heat

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Accurate investigation of the relationship between heat developed and mechanical work spent was taken up by the British Scientist James Prescott Joule. The main aim of his investigations was to determine exactly the ratio between the work done and the quantity of heat produced.

Joule used different arrangements for doing the work, W in different ways and measured the corresponding amount of heat, H produced in each case. In all the cases he found that the expenditure of the same amount of work always produced the same amount of heat. Every time he found that 4186 joule of work was spent to produce the same amount of heat which could raise the temperature of one kg of water through 1⁰C. He established the relation, W/H to be a constant quantity. The constant relation W/H was represented symbolically by the letter ‘J’. J is known as Joule's mechanical equivalent of heat. ‘Symbolically, we can write :

W/H = J or W = JH

Thus he established that heat is a form of energy

Details of Apparatus : The apparatus consists of a specially designed calorimeter placed in a wooden box C with felt lining to avoid heat losses to the surroundings. A number of vanes, V, V projects from the walls of the calorimeter in its interior. A spindle carrying a number of brass paddles P, P acts as a churner and it is so pivoted at the bottom that the paddles P, P are capable of turning between the fixed vanes V. The spindle can be attached to a drum D by means of screw S’ whenever desired. The drum D can be rotated by the handle H or by the falling weights W, W, which are attached to two pieces of string passing over the pulleys P₁ and P₂ and wrapped round the drum. Two vertical scales are fixed to note the vertical distances through which the weights fall.

Working : The two weights W, W are allowed to fall through a height ‘h’. In doing so, they turn the spindle and thereby turn the paddles P, P immersed in a known mass (m) of water contained in the calorimeter. The water is thus churned but not allowed to rotate due to fixed vanes and the potential energy of the falling weights is converted into kinetic energy of paddles. Due to friction offered by paddles KE is changed into heat and as a result of it, the temperature of the water in the calorimeter rises. The rise in temperature is measured by an accurate thermometer T inserted in the calorimeter. The process is rapidly repeated several times (every time detaching the drum, wrapping the string, tightening the screws’ and allowing the weights to fall), (say n times) such that there is an accurately measurable rise in the temperature of water.

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Importance of Joule’s Experiments

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Joule’s experiments conclusively established that heat is a form of energy. It is not a material substance like caloric fluid.
Always the same amount of heat was produced by spending a given amount of mechanical work. It is immaterial what type of arrangement is used for doing mechanical work. Other alternative ways used by Joule for conversion of mechanical work into heat were (i) by mechanically stirring mercury, and (ii) by rubbing two iron rings together. Every time he found that W/H is a constant quantity, i.e., when 4.186 Joule converted to heat the temperature of 1 kg of water will rise by 1⁰C.

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