ELECTROMETERS. 229 



that is the difference of potentials between the jaT and the 

 cage or the earth, is measured by the square root of the 

 number of degrees of torsion. When I say that the 

 potential of the jar is measured by the square root of the 

 number of degrees of torsion, I mean that if the jar be 

 electrified to different potentials the square roots of the 

 torsional readings will be in proportion to those potentials. 



Now, the charge of the jar being kept unaltered, the 

 electrode of the cage is disconnected from the earth and 

 connected with the conductor to be tested, and the cage 

 thus acquires the same potential as the conductor to be 

 tested. The electrification of the cage causes an alter- 

 ation in the force acting upon the needle which moves 

 towards or from the repulsion plates, according as the 

 electrification of the cage is similar or dissimilar to that 

 of the jar. The torsion head is now turned so as to 

 bring the needle back to the sighted position ; and the 

 torsion is again read off. As before, the square root of 

 the number of degrees of torsion measures the difference 

 of potentials between the cage, and the needle, that is also 

 the difference of potentials between the conductor tested 

 and the needle. The excess, positive or negative, of the 

 square root of this last reading above the square root 

 of the "earth reading," is the difference of potentials 

 between the body tested and the earth ; and if, as is com- 

 monly done, we regard the potential of the earth as zero, 

 it is simply the potential of the conductor tested. 



This electrometer has done good service in observations 

 on atmospheric electricity. Indeed it was first constructed 

 for that purpose. The electrode of the cage is connected 

 with a collector of electricity, either with a flame-collector, 

 as first described by Volta, or with the water dropping 

 collector of Sir William Thomson, and earth readings and 

 atmospheric readings are taken alternately at proper inter- 

 vals. The numbers obtained for the differences of potentials 

 are reduced to absolute measure by comparison with a few 

 cells of a known battery; but this I will explain a little 

 later. 



[A form of portable electrometer, also due to Thomson, 

 was next exhibited and described. The description is not, 

 however, reproduced here, as the instrument differs little 

 in electrical principles from that which has just been 



