510 The Rev. T. R. Robinson's Experimmtal Researckes on the 



a pair of spirals (^), possessing a definite character, and acting on the magnet 

 under the most favourable circumstances, namely, when its cylinders are reduced 

 so as merely to lodge the wire, in which case the action with a given ^ is the 

 greatest possible. With these I obtained a series of values of L and the cor- 

 responding -^'j from which can be found, by interpolation, the ^' correspond- 

 ing to any L. If that L is obtained by any other spirals, helices, or altered 

 length of the magnet, we have, assuming the mean efficiency of (^4) = 1, 



As, however, in using different currents, the magnet is unequally heated, it is 

 necessary, in the first instance, to determine 



THE TEMPERATURE CORRECTION. 



In my former paper I investigated the correction by heating the magnets to 

 about 70'' and 170' ; and, assuming that the decrease was uniform throughout 

 this interval, I deduced the coefficient of decrease = 000033. The experi- 

 ments of Dr. Lloyd on the temperature change of the magnetism induced by 

 the earth on soft iron have led me to doubt the correctness of this assumption, 

 and institute further experiments, which show that the law is much more com- 

 plicated in the case of the electro-magnet, and that the coefficients which ex- 

 press it vary with the nature of the magnet. The magnet used in the first 

 instance is one belonging to ]\Ir. Bergin's collection (to whom I am indebted 

 not only for the use of much valuable apparatus, but for still more valuable 

 assistance in these researches), extremely convenient for the work. The cylin- 

 ders are five and a half inches diameter, their centres six asunder ; they are 

 hollow, their thickness being half an inch ; they are eight inches long, and the 

 base and keeper have a section equal to theirs. The helices are those desig- 

 nated (/). The balance used with it is well worth notice. It has only one 

 lever (whose ratio is 11625), the longer arm of which carries a platform, on 

 which weights, multiples of 30 lbs., can be placed : below the platform is sus- 

 pended, by a spring balance, a tin vessel, into which shot is poured, whose 

 weight is seen by the index up to 30 lbs., the limit of the spring. If this be 

 not sufficient, the shot is permitted to escape by a valve at the bottom of the 

 vessel, another weight is set on the platform, and the process is repeated. The 



