28 



NA TURE 



[May 8, 1884 



what is now commonly spoken of as the back electro- 

 motive force of a motor. In connection with these re- 

 searches Mr. Joule obtained valuable results with regard 

 to the construction and the efficiency of various forms of 

 magnets, both permanent magnets and electro-magnets, 

 and he was led also to improvements in the galvano- 

 meter which, in the form of the tangent galvanometer, 

 he afterwards perfected. 



These experiments led naturally to investigations on 

 the connection between heat, electricity, and mechanical 

 energy, and to a comparison between electricity obtained 

 from chemical action and that obtained from magneto- 

 electric machines, and also to an examination into the 

 heat given out during electrolysis. 



A paper of March 1841, on the heat evolved by metallic 

 conductors of electricity and in the cells of a battery 

 during electrolysis, is of special interest. It is here that 

 the law is first announced that the heat developed by a 

 current of electricity, whether through a metallic con- 

 ductor or in an electrolytic cell, is proportional to the 

 resistance and to the square of the current. It is proved 

 that the whole heat generated by a voltaic battery is pro- 

 portional to the chemical action which goes on in each cell 

 of the battery multiplied by the whole "intensity" or 

 electromotive force ; and the localities at which the 

 several portions of the heat developed in a compound 

 circuit, are clearly distinguished, and the quantities of 

 heat developed in each part are determined. 



In this paper improvements in the galvanometer are 

 referred to. A "degree of electricity," or unit-current, 

 is defined as "the quantity of current electricity which is 

 able to electrolyse a chemical equivalent expressed in 

 grains in one hour of time." Hence the results in this 

 paper, and in many others which follow and in which 

 the same degree is used, are now easily reducible to abso- 

 lute measure. His degree was somewhat less than two 

 amperes, or one-fifth of the absolute C.G.S. unit. In this 

 paper also he defines his first unit of resistance — a wire 

 of copper ten feet long and 0^024 of an inch in diameter 

 (about No. 23 B.W.G.) ; and it is curious and somewhat 

 amusing to find that the copper wire which Joule used 

 for this unit must have been preternaturally bad! If 

 the wire had been of " conductivity " copper, such as is 

 now universally insisted on, the resistance would have 

 been C167 ohm. An easy calculation from Joule's re- 

 sults shows that the resistance must have been at least 

 one-half more ! It was not until the manufacture of the 

 1858 Atlantic cable was in progress that it wasfound 

 that variations, not previously dreamed of as possible, 

 were commonly to be met with in the conductivity of 

 copper wire. 



A most interesting paper on the electric origin of the 

 heat of combustion, also in 1841, naturally follows that 

 just referred to. It is in this paper that Joule determines 

 the electromotive force necessary to decompose water. He 

 rinds it to be 2 -8 of Smee's elements, and then proceeds 

 to similar determinations for various chemical compounds 

 used as electrolytes. 



Space fails altogether for mentioning the multitude of 

 interesting results, then perfectly unknown, which Joule 

 brings out in these early papers. Many of them have 

 played important parts in guiding and in assisting other 

 investigators. We find tests recorded as to permanency 



of resistance coils. We have investigations of the resist- 

 ance of electrodes of various materials in various electro- 

 lytic cells. Joule's early (1841 to 1844) determinations 

 enabled Sir William Thomson in 1851 to calculate in 

 absolute measure the electromotive force of a Daniell's 

 cell. He found it to be 2,507,100 British absolute units or 

 1 '0739 v0 '' '■ I' > s doubtful whether we are assured of a 

 better result at the present day. 



We must notice next the series of papers containing 

 Joule's researches on the dynamical equivalent of heat, 

 unquestionably the most important of all his investiga- 

 tions. The complete and successful prosecution cf 

 this investigation belongs to Joule, and to Joule alone. 

 The methods are his ; the carrying out of the experiments 

 is his. The result will ever be known under the honoured 

 name of "Joule's equivalent." 



It is interesting to notice the first germ of the idea, and 

 to be enabled to follow, from its commencement to its 

 conclusion, the series of experiments which gradually 

 brought out the result with which we are now so well 

 satisfied. 



In a paper dated January 24, 1843, we find the first 

 mention of the idea as follows : — 



" The magnetic electrical machine enables us to con- 

 vert mechanical powers into heat by means of the electric 

 currents which are induced by it. And I have little doubt 

 that, by interposing an electro-magnetic engine in the 

 circuit of a battery, a diminution of the heat evolved per 

 equivalent of chemical change would be the consequence, 

 and this in proportion to the mechanical power obtained." 



A note dated February iS, 1843, is as follows :— 



" I am preparing for experiments to test the accuracy 

 of this proposition." 



The results of the experiments alluded to in the note 

 just quoted were given to the British Association at its 

 meeting at Cork, in a paper read on August 21, 1843, " On 

 the Calorific Effects of Magnetic Electricity, and on the 

 Mechanical Value of Heat." The experiments were 

 made by rotating " an electro-magnet immersed in a vessel 

 containing water between the poles of a powerful magnet, 

 to measure the electricity thence arising by an accurate 

 galvanometer, and to ascertain the caloric effect of the 

 coil of the electro-magnet by the change of temperature 

 in the water surrounding it.'' 



Permanent steel magnets were first employed for pro- 

 ducing the magnetic field, and afterwards a huge sta- 

 tionary electro-magnet was used for this purpose. The 

 writer of the present notice well remembers the interest 

 with which this great rough magnet and its accompani- 

 ments were viewed, by some of the foreigners who visited 

 the Loan Collection of Scientific Apparatus at South 

 Kensington in 1877. 



Joule's conclusion, given to the British Association at 

 this time was that the mechanical equivalent of a water 

 pound-degree Fahrenheit of heat was 838 foot-pounds of 

 work. In a postscript to this paper, of date August 1843, 

 he says : — 



" I have lately proved experimentally that heat is 

 evolved by the passage of water through narrow tubes, 

 My apparatus consisted of a piston perforated by a 

 number of small holes working in a cylindrical glass jar 

 containing about 7 lbs. of water. I thus obtained one 

 degree of heat per pound of water from a mechanical 

 force capable of raising about 770 lbs. to the heigh of 



