2 Prof. G. Quincke on the Constant of Dielectricity 
pendicular and parallel to the lines of electric force may vary 
according to the nature of the insulating substance. 
I have measured, for a series of insulating fluids, the value 
of K and K x for the pulling and pushing forces parallel and 
perpendicular to the lines of electric force. I will denote K } 
by K p or K s , according to whether the action of the forces is 
parallel or perpendicular to the lines of electric force. 
I employed in those measurements an electric balance with 
plane horizontal condenser-plates of nickel-coated brass 8*530 
centim. in diameter and distant 0"1597 centim., which stood 
in a larger glass vessel filled with air or the insulating fluid. 
The lower condenser-plate was connected with the inner coat- 
ing of a large Leyden battery of eight flint-glass jars, the outer 
coating of which, as well as the upper plate of the condenser, 
was conducted to earth. 
The plates of the condenser and the coatings of the battery 
were charged by a Holtz machine, and were maintained at a 
constant difference of potentials. The latter could be con- 
trolled by a reflecting electrometer described by M. Righi *, 
or measured by a screw-electrometer (long-range electrometer) 
of Sir William Thomsonf. 
A comparison of the indications of the screw-electrometer 
with the measurements in an electric balance in air, with con- 
denser-plates of 85 millim. diameter and at from 1 to 2 millim. 
distance, showed that to one turn of the screw of the electro- 
meter (of which O001 revolution could be determined by 
estimation) corresponded an alteration of the electric potential 
amounting to 1-1415 C.G.S., or the electromotive force of a 
Daniell battery of 305 cells. 
With the screw-electrometer I determined the difference of 
electric potentials at which the condenser-plates in air attracted 
each other with a force Gi of 20 or 10 grammes. 
I next sought to ascertain the weights which, with equal 
difference of electrical potentials, exactly counterbalanced the 
electric attraction of the condenser-plates (parallel to the lines 
of electric force) when the plates were entirely surrounded by 
the insulating fluid instead of air. These weights, Gn, varied 
between 21 and 100 grammes, and could be determined with 
precision to 0*01 grm. by a sensitive balance. The ratio of 
the weights Gn and Gi found for the fluid and air, with equal 
electric potential-difference, gives K p . 
When the condenser-plates of the electric balance were 
charged up to the same difference of electric potentials, and 
* Memorie deW Accademiu delle Scieuze delV Istituto di Bologna. [31 vii. 
2, p. 193 (1876). 
t Papers on Electrostatics and Magnetism, p. 300, pi. ii. tig. 15. 
