Mr. A. P. Chattock on a Magnetic Potentiometer. 95 

 the angle 6 such that 



Vi-V^Ksinl 



and the combination forms what may be called a magnetic 

 potentiometer. 



In exploring a permanent field with such an apparatus, the 

 best way is, perhaps, to fix one end of the helix in a clip, 

 thereby keeping its potential constant, and to move the other 

 end from point to point in the field. For this purpose the 

 wire should be wound upon a flexible core, the average length 

 of which, whether bent or straight, must be constant (other- 

 wise da . dn will not be equal to n . dv in the last equation). 



I have therefore constructed a helix by winding wire uni- 

 formly on to a piece of solid indiarubber, of constant cross 

 section (in my case 37 centim. long and about 1 centim. dia- 

 meter), leaving a small space between one turn and the next 

 to allow the indiarubber to bend without elongating. 



With this apparatus I made the following measurements of 

 the potential difference between the ends of a permanent bar- 

 magnet, in order to test the accuracy of the method. In the 

 first set of readings the free end of the helix was moved at one 

 leap from end to end of the magnet, giving a mean reading 

 of 6'017 ; in the two other sets it was moved between the 

 same two points in two and four leaps respectively, the result- 

 ing readings being added together in each case. The means 

 of these were 6*047 and 6*048. The final mean was thus 

 6*037; and from this the most discordant reading differed by 

 about 1 per cent. The close agreement between the second 

 and third values was, no doubt, accidental ; but the difference 

 between them and the first may have been due to the diffi- 

 culty of moving quickly from end to end of the magnet, a 

 distance of over 40 centim. 



In order to reduce these results to absolute measure, the 

 helix (still connected with the galvanometer) was subjected 

 to a known magnetomotive force by passing it through a coil 

 of n turns and bringing its ends close together outside the 

 coil, a current C being then started or stopped in the latter. 

 The magnetomotive force due to this was of course equal to 

 47rnC, and this being substituted for Vj— Y 2 in the last equa- 

 tion determined the value of K. In my case these values 

 were rc = 20, = 0*182 C.Gr.S., and galvanometer throw =0*34. 

 The difference of potential between the ends of the magnet 

 was thus 



^rx 20x0-182 

 0*34 



