666 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
the two forks, and for the outstanding bridge-wire divisions, the necessity for which dis¬ 
guises the significance of the numbers first obtained, it was found that the agreement 
of the results corresponding to a given speed was by no means so good as we had 
expected in view of the precautions taken and the accuracy of the readings. What 
was worse, there was evidence of a decided progression, as if the absolute resistance 
of the standard had gradually diminished during the time occupied by the spinnings. 
It is not impossible that there really was some change in the standard which had 
then been newly prepared; but the discrepancies were not, as according to this view 
they ought to have been, proportional to the speeds of rotation. I am inclined rather 
to attribute them to shiftings of the paper scales. The principal magnetometer scale 
was composed of three lengths of 50 centims. each, cemented with indiarubber to a 
strip of deal. The compound scale thus formed was examined by Dr. Schuster in 
March, 1881. Between the graduations of the first and of the middle piece there was 
a gap of about -J- millim., and another of nearly the same magnitude between the 
middle and the third piece. When I re-examined the scale in July, the gap at 500 
divisions had increased to millim., and that at 1000 to \ millim. Curiously enough, 
there were no observable errors in the equality of the divisions of the three parts 
taken separately ; but the changes above-mentioned are sufficient to throw considerable 
doubts upon the value of the first series of spinnings. They have, however, been 
reduced by Dr. Schuster, and the result is given below for the sake of comparison. 
To be free for the future from uncertainties of this kind, I replaced the paper scale 
by a long glass thermometer tube by Casella, graduated into millimetres. The 
divisions were fine and accurately placed, but the imperfect straightness of the tube 
has rendered necessary certain small corrections in the final results. Probably a 
straight strip of flat opal would have been an improvement. 
The second series of spinnings was made in August, 1881, and this, it was fondly 
hoped, would be final. To guard against possible change in the platinum-silver coil a 
careful comparison with the standard units was previously instituted by Mrs. Sidgwick, 
of which the details are given later. As we had unfortunately lost the advantage of 
Dr. Schuster’s assistance, the observations at the principal magnetometer devolved 
upon Mrs. Sedgwick. The much easier post at the auxiliary magnetometer was 
usually occupied by Lady Bayleigh ; occasional assistance has been rendered by 
Mr. A. Mallock and by Mr. J. J. Thomson. 
In the conduct of the second series one or two minor changes were introduced. In 
order to know the temperature of the standard tuning-fork more accurately, a thermo¬ 
meter was placed between its prongs and read at the same time as the number of beats 
was taken. The insertion of the small resistances necessary to bring the copper coil 
within range of the standard was also arranged in a different manner. Some trouble 
had been experienced in getting a sufficiently good fit between the contact pieces used 
in the first series and the mercury cups. It is necessary that the stout copper terminals 
should press down closely upon the bottoms of the cups, and also that the mercury 
