198 Mr. E. B. Rosa on the Specific 



to the force, we have the following expressions for the tem- 

 perature-coefficients : — 



For Alcohol, g . — =: --0046 for T=20° C. 

 For Water, ~. ^= --0038 for T=20° 0. 



Values of K. 



IV. The value of K for any liquid may be determined by 

 taking the ratio of the force on the needle at a given distance 

 from the fixed electrode for a given potential, the medium 

 being the given liquid, to the force on the needle for the same 

 distance and potential, the medium being air. Since the 

 needle lies wholly upon one side of the axis of suspension, the 

 latter is slightly displaced when the needle is immersed in a 

 liquid, owing to the buoyancy of the liquid. This displace- 

 ment is in a vertical plane passing through the centre of 

 gravity of the whole suspended mass and the centre of 

 gravity of the displaced liquid ; and if this plane is not 

 parallel to the plate, the distance between needle and plate 

 will be thereby slightly altered. To avoid any error due to 

 such a cause the needle was placed between two similar plates 

 15 millimetres apart, and the double deflexions noted as the 

 needle was alternately joined to either plate, the other being 

 at a different potential. The double deflexion was found to be 

 constant for any position of the needle near the centre, so 

 that no error could arise from small displacements due to the 

 above cause. The results showed a slightly different value 

 for the ratio sought than that found with a single plate, but the 

 difference was so small that no appreciable error is believed 

 to occur in comparisons of liquids, due to different specific 

 gravities. 



The needle w r as so constructed that in water, alcohol, or 

 light oils it was nearly dead-beat while yet moving quickly. 

 A brass weight attached just below the mirror rendered the 

 suspension very stable, and when air- currents were excluded, 

 the needle would take up a perfectly definite zero-position, to 

 which it would usually return without a deviation of a 

 thousandth of a millimetre. To render the needle equally 

 stable and dead-beat in the air, a special damper (a vane of 

 platinum hanging in oil) was attached in the axis of rotation. 

 Diamond White headlight oil * was chosen as a favourable 



* This is a high grade of refined petroleum. 



