438 KEPOET — 1882. 



satisfied for a time with work of a less severe and accurate character, knowing that 

 with the progress of knowledge the way is sure to be smoothed both by a better 

 appreciation of the difficulties involved, and by the invention of improved experi- 

 mental appliances. I hope I shall not be misunderstood as underrating the 

 importance of great accuracy in its proper place if I express the opinion that the 

 desire for it has sometimes had a prejudicial effect. In cases where a rough result 

 would have sufficed for all immediate purposes, no measurement at all has been 

 attempted, because the circumstances rendered it unlikely that a high standard of 

 precision could be attained. AVhether our aim be more or less ambitious, it is 

 important to recognise the limitations to which our methods are necessarily subject, 

 and as far as possible to estimate the extent to which our results are uncertain. 

 The comparison of estimates of uncertainty made before and after the execution of 

 a set of measurements may sometimes be humiliating, but it is always instructive. 



Even when our results show no greater discrepancies than we were originally 

 prepared for, it is well to err on the side of modesty in estimating their trustworthi- 

 ness. The history of science teaches only too plainh' the lesson that no single method 

 is absolutely to be relied upon, that sources of error lurk where they are least ex- 

 pected, and that they may escape the notice of the most experienced and conscien- 

 tious worker. It is only by the concurrence of evidence of various kinds and from 

 various sources that practical certainty may at last be attained, and complete con- 

 fidence justified. Perhaps I may be allowed to illustrate my meaning by reference 

 to a subject which has engaged a good deal of my attention for the last two years — 

 the absolute measurement of electrical resistance. The unit commonl}' employed in 

 this country is fomided npon experiments made about twenty years ago by a dis- 

 tinguished committee of this Association, and was intended to represent an absolute 

 resistance of 10^. C.G.S., i.e. one ohm. The method employed by the committee 

 at the recommendation of Sir W. Thomson (it had been originally proposed by 

 Weber) consisted in observing the deflection from the magnetic mea-idian of a needle 

 suspended at the centre of a coil of insulated wire, which formed a closed circiut, 

 and was made to revolve with uniform and known speed about a vertical axis. 

 From the speed and deflection in combination with the mean radius of the coil and 

 the nimiber of its turns, the absolute resistance of the coil, and thence of any other 

 standard, can be determined. 



About ten years later Kohlrausch attacked the problem by another method, 

 which it would take too long to explain, and arrived at the result that tlie B.A. 

 unit was equal to 1'02 ohms — about two per cent, too large. Rowland, in 

 America, by a comparison between the steady battery current flowing in a primary 

 coil with the transient current developed in a secondary coil when the primary 

 current is reversed, found that the B.A. unit was 'OOl ohms. Lorentz, using a 

 different method agaiu, found -OSO, while H. "Weber, from distinct experiments, 

 arrived at the conclusion that the B.A. unit was correct. It will be seen that 

 the results obtained by these highly competent observers range over about four 

 per cent. Two new determinations ha^e lately been made in the Cavendish 

 laboratory at Cambridge, one by myself with the metliod of the revolving coil, and 

 another by Mr. Glazebrook, who used a modification of the method followed by 

 Rowland, with the result that the B.A. unit is -986 ohms. I am now engaged 

 upon a third determination, using a method which is a modification of that of 

 Lorentz. 



In another important part of the field of experimental science, where the 

 subject-matter is ill imderstood, and the work is qualitative rather than quantita- 

 tive, success depends more directly upon sagacity and genius. It must be admitted 

 that much labour spent in this kind of work is ill directed. Bullty records of crude 

 and uninterpreted observations are not science, nor even in many cases the raw 

 material out of which science M'ill be constructed. The door of experiment stands 

 always open ; and when the question is ripe, and the man is found, he wiU nine 

 times out of ten find it necessary to go through the work again. Observations 

 made by the way, and imder unfavourable conditions, may often give rise to valuable 

 suggestions, but these must be tested by experiment, in which the conditions are 

 simplified to the utmost, before they can lay claim to acceptance. 



