512 REPORT— 1867. 



§ 44. The heterostatic principle in one form or other is essential to distin- 

 gnish between positive and negative. As remarked above (§ 42), the original 

 type of this use of it is to be found ia the old system of testing the quality 

 of the charge taken by the diverging straws or gold leaves of the electroscopes 

 used for the observation of atmospheric electricity ; which was done by 

 bringing a piece of rubbed sealing-wax into the neighbonrhood, and observ- 

 ing whether this caused increase or diminution of the divergence. A donbt 

 which still exists as to the sign* of the atmospheric electricity observed by 

 Professor Piazzi Smyth on the Peak of Teneriffe, is owing to the imperfection 

 of this way of applying the principle. It is, indeed, to be donbted in any one 

 instance whether it is not vitreous electricity that the rubbed sealing-wax 

 acquires. And, again (§ 2), it is not certain that the glass case enclosing the 

 gold leaves, especially if very clean and surrounded by a very dry natural 

 atmosphere, screens them sufticiently from direct influence of the piece of 

 sealing-wax to make sure that the divergence due to vitreous electricity could 

 not be increased by the presence of the resinously electrified seahng-wax if 

 held nearer the gold leaves than the npper projecting stem. 



§ 45. The heterostatic jn-inciple lias a very great advantage as regards 

 sensibility over any simple idiostatic arrangement, inasmuch as, for infinitely 

 small differences of potential to be measured, the force is as the squares of 

 the differences in any idiostatic arrangement, but is simply proportional to 

 the differences in every heterostatic arrana'cment. 



VI. Determination of ilte Dynamical Equivalent of Heat from the thermal 

 effects of Electric Currents. By J. P. Joule, D.C.L., F.B.S., 4'c. 



Sir W. Thomson, as long ago as 1851, showed that it was desirable to 

 make experiments such as arc the subject of the pi'esent paper. They have 

 necessarily been delayed until a sufficiently accurate method of measuring 

 resistance was discovered. Such a method having been described bj' Sir 

 William, and carried out into practice by Professor C. Maxwell and his able 

 coadjutors, the task assigned to me by the Committee of Electric Standards 

 was comparativeljr simj^le. 



My experiments were commenced nearly two years ago, and the apparent 

 ease with which they could be executed gave promise of their early com- 

 pletion. It was, however, found essential that careful observations of the 

 earth's horizontal magnetic intensity should be frequently made, and these 

 required the construction of apparatus whereby this element could be deter- 

 mined with acci;racy and rapidity. 



The apparatus finally adopted for this purpose consists of a suspended 

 horizontal flat coil of wire between two fixed similar coils. A current of 

 electricity can be made to traverse all three, communication with the sus- 

 pended coil being made by the suspending wires themselves according to Sir 

 "W. Thomson's plan. The sti-ength of a current is found by observing the 

 sum of the forces of attraction and repulsion by which the suspended coil is 

 urged. The strength of a current can in this manner be determined in ab- 

 solute measure. For the area of each of the three equal coils being called 

 a, the weight required to coimterpoise the force with which the suspended one 

 is urged w, the force of gravity g, and the length of wire in each of the coils 



Z, the current c=Z)Ja/ -^^ (1 -|- coi rection), the correction being principally 



* Niclior.s Cyclopredia, article " Electricily. Atmospheric," edition ISCO. 



1 



