A NEW CURRENT UTIGHER, ETC. 539 



1 part in 70,000. The difference between the means of the 26 and the 71 deter- 

 minations is 1 in 1,000,000. 



These results are of considerable importance, as they show very great constancy 

 both of current weigher and cell. In fact, the cell and balance proved to be much 

 more constant and reliable than the standard resistance, although the latter was very 

 carefully made and annealed with a view to ensuring permanency. 



The precision of measurement attainable with the new balance exceeds the most 

 sanguine expectations of its designers. It was intended to give the ampere to 1 in 

 10,000, and an accuracy of 1 in 20,000 was hoped for, but 1 in 50,000 has been 

 attained. The instrument itself admits of a far higher accuracy, for a tenth of a 

 milligramme can be detected with certainty, and this, in a total of 15 grammes, the 

 balancing mass for 1 ampere, means 1 in 300,000 in the value of the current. This 

 is a precision considered to be of a very high order, even for relative measurements. 

 Uncertainty, however, exists as to the value of g, and the axial lengths of the coils, 

 which prevent the highest accuracy of which the balance is capable, being realised at 

 present. 



Directions in which improvements may be looked for are, therefore : 



(i) A more accurate determination of the acceleration due to gravity, and 

 (ii) Greater precision in the means for measuring the axial lengths of the coils, or a 

 lengthening of the coils to reduce the effect of this possible error. 



As the uncertainty in g is of most consequence, we trust that an absolute deter- 

 mination of its value at the National Physical Laboratory will, ere long, be made. 



To realise the volt to an accuracy approaching that of the ampere as now known, 

 it is necessary that an absolute determination of resistance of corresponding precision 

 be undertaken. At the present time the uncertainty in the absolute value of the 

 international ohm, in terms of which our values of C x R for the cadmium cells are 

 expressed, approximates to 4 in 10,000, so it is of considerable importance that a 

 better determination be made at an early date. 



In conclusion, we desire to express our sincere thanks to the British Association for 

 providing the funds with which to construct the ampere balance, and to Sir ANDREW 

 NOBLE, F.R.S., for presenting the adjustable stand to support the instrument. 



Our most hearty thanks are hereby tendered to Dr. R. T. GLAZEBKOOK, F.R.S., 

 Director of the National Physical Laboratory, for supervising the construction of the 

 electrical portions of the balance, for the keen interest he has taken in the experi- 

 ments, and also for having placed the very perfect resources of the Laboratory at our 

 disposal. Indeed, much of the precision attained in the results is due to the facilities 

 available at the National Physical Laboratory for such work. To Dr. T. E. STANTON 

 we are indebted for superintending the turning of the marble cylinders used to 

 support the coils of the balance. 



Our best thanks are also due to Mr. J. P. GREGORY for valuable assistance rendered 



8 z 2 



