the Magnetic Kffect of Electric Convection. 455 



xlonce / = -, — . 



Let r be the total olinnc resistance of the conducting ring. 



Then the resistance of NRN, is - and that of NPNj is r . 



The conduction- current in NRNj is evidently 



---'■f^> 



The magnetic field at a distance above R great with 

 reference to p will be due to the ditference between i and i^, 

 that is. to 



i^-'-^h* 



Similar reasoning shows that the field over P is propor- 

 tional to . 



n 



Hence, a magnetic system placed over P or R will show 

 magnetic effects equal and in opposite directions. 



Rowland tried this experiment, but the smallness of the 

 effect expected prevented his obtaining satisfactory results. 



He tried the experiment, giving - the value 3. The con- 

 ducting ring was thin gold-leaf on an ebonite disk, the 

 width of the rino- beino- 5 cm. and the mean radius l^'o cm. 

 The calculated intensity of the convection-current was 



4 X 10~'^ ampere. The deflexions of an astatic system 

 placed over P were in the direction expected and accorded 

 quantitatively with the calculated deflexion within the usual 

 approximation in our experiments, i. e., 10 to 15 per cent. 



However, by slightly modifying the experiment we were 

 able to measure the intensity of the convection with great 

 precision. If the two points on the disk immediately under 



5 and Si are connected to a galvanometer the resistance of 

 which is of the same order as that of the halves of the ring 

 NRMP, there will flow through the galvanometer a current 

 easily measurable, since it will be a considerable fraction of 

 the convection-current, that is to say, in this experiment, of 

 the order of 10 ~'^ ampere. The gilding on the disk can be 

 made extremely thin, having a resistance from 4 to 6 ohms 

 per square centimetre of surface. The resistance of one-half 

 the ring on our disk was 6 ohms. We employed a galvano- 

 meter of the d'Arsonval type (Hartmann and Braun) having 



