262 Messrs Burton and Phillips, 



horizontal. These were fixed once for all at a distance of 1*2 cms. 

 apart. As the slab (A ) was over 8 cms. long, H£ may be neglected 

 in comparison with H 1 , so that the expression for P becomes 



k.AH* 



In order to measure k for a liquid, the slab A is replaced by 

 a small rectangular glass vessel, and the difference between the 

 force on the empty vessel and that on the vessel full of liquid, 

 gives the pull on a known slab of liquid. The internal dimensions 

 of the vessel used were 3*01 cms. by *873 cms. by 8 - cms., of 

 which only the dimensions of the horizontal section need be 

 known accurately. The vessel was filled through a small hole 

 in the top, which could then be sealed up so as to prevent 

 evaporation. 



The colloidal solution was prepared by sparking (40 volts, 

 6*5 amps.) between two soft iron electrodes beneath the surface 

 of pure methyl alcohol, which was kept surrounded with ice. In 

 order to estimate the quantity of iron per c.c. in such a solution 

 a measured quantity was evaporated in a previously weighed 

 porcelain dish ; the residue was then heated to redness so as to 

 convert it to Fe 2 3 and weighed. 



Experimental Results. 



The results will be tabulated under the following heads : 

 (1) the measurement of the field between the poles, (2) the 

 susceptibility of pure methyl alcohol, (3) the susceptibility of the 

 colloidal solution, and (4) the deduction of the Constant in the 

 Townsend formula. 



1. Measurement of Field. 



The field was measured by finding the pull exerted by it upon 

 a conductor carrying a known current, and placed horizontally 

 between the poles. A block of wood having the same external 

 dimensions as the glass cell to be used, was made so as to permit 

 it to hang between the poles in exactly the same position as the 

 cell itself. Around the two vertical edges and bottom of this 

 block was pasted a continuous strip of tin-foil ; to the upper ends 

 of the foil wires were soldered, and then led out so that any 

 desired current might be passed through the foil. If the current 

 carried by the foil be i amperes, the length of the horizontal 

 portion of the foil be I cms., then the pull, P, in grams exerted by 

 a field (H) on the lower horizontal strip of the foil will be given 

 by the equation 



P-™ ie H- 10Pg 

 10g' li • 



