512 KEPORT — 1889. 



to equilibrium when disturbed by the passage of the current. A sliding weight 

 may also be used as in a steelyard. 



It will be readily seen that, as in all forms of current-weigher, the weights 

 are proportional to the square of the current strength. 



9. 071 a Phenomenon in the Electro-chemical Solution of Metals. 

 By Professor S. P. Thompson, Ph.D. 



10. On the Employment of Chromic Acid instead of Nitric Acid in the 

 Bunsen Cell. By J. Wilson Swan. 



Since chromic acid has come into the market at a low price experiments have 

 been made with it with a view to find a non-fLiming depolariser capable of 

 replacing nitric acid in the Bunsen cell. It is found that the following mixture is 

 very nearly equal to nitric acid in this respect : — 



Parts by Weight 



Nitric acid s.g. 142 1 



Chromic acid 3 



Sulphuric acid 6 



Water 5 



11. A Variable Standard of Self-induction. 

 .By Professor J. Perky, F.P.S. 



An instrument like that hitherto used as a variable standard by Professor 

 Ayrton and the author was used by Professor Hughes and by Lord Rayleigh. It 

 is a fixed coil of wire, inside which a movable coil is placed. The coils are in series 

 ■with one another. When their planes are parallel there is either a minimum or a 

 maximum coefficient. When the movable coil is rotated so that its plane makes 

 various angles with the plane of the fixed coil, a pointer shows on a scale the co- 

 efficient for that particular position. In the specimen hitherto used the range 

 was very small, but in the specimen exhibited the minimum coefficient was only 

 one-sixth of the maximum, and by winding with finer or coarser wire the readings 

 could be increased or reduced to any extent. 



As it was unlikely that any other person would take the trouble to have such 

 standards made, the author hoped for hints at its improvement. It was possible 

 to reduce the lowest reading to zero by introducing a condenser of such a capacity 

 K, that if L is the minimum coefficient of self-induction and R is the resistance, 



L = KR- 



according to Mr. Sumptuer's law concerning a shunted condenser. This law only 

 concerns the case of currents which change from one steady value to the other, 

 and the author had worked out a number of numerical examples showing the error 

 introduced by this assumption when the currents were alternating as sine functions 

 of the time. Two unknowns had to be determined if both the amplitude and lay 

 ■weve to be the same with tbe condenser as with lessened self-induction, and there- 

 fore there ought to be a coil of wire in series with the condenser. He wanted 

 advice as to whether the introduction of the condenser or the use of an independent 

 second variable standard was the better. 



12. Hot Twisted Strip Voltmeter. By Professor J. Pekey, F.R.8. 



The author described the behaviour of twisted strips subjected to axial pull. 

 A small elongation is accompanied by a great rotation, so that these strips maybe 

 employed in measuring instruments such as weighing machines, aneroid barometers, 

 testing machines, &c. For the Ayrton and Perry voltmeter a double-twisted 

 strip of platinum silver with its ends insulated is initially in tension inside a tube 

 or frame two-thirds brass and one-third iron, a pointer at the middle of the strip 



