CONTINUOUS ELECTRIC CALORIMETRY. 
71 
employing longer columns in the case of a mercury thermometer is that the errors 
of estimation become of relatively less importance. It is practically possible to 
measure both the long and short columns to the same fraction of a degree, and an 
error of ‘001° in a column of 20° is of much less relative importance than in a 
column of 2°. In the case of the resistance box, on the other hand, the relative 
accuracy of measurement is undoubtedly greater in the case of the smaller 
resistances. A 10,000-ohm coil cannot be measured with the same order of accuracy 
as one of 1000 ohms. This is partly due to difficulties of insulation in the winding 
of the coils themselves, and partly to the fact that wire finer than 2 or 3 millims. 
cannot be drawn and covered satisfactorily. As a consequence, high-resistance 
galvanometers are necessarily less efficient than low-resistance instruments of similar 
construction. The best high-resistance coils are constructed of a number of lower 
resistance coils in series, as in the Thomson-Varley slide-box, which permits a higher 
order of insulation than winding in a single coil. 
(B.) Resistance. 
(6.) 'The Lorenz Apparatus. 
Although this apparatus was not actually applied to the direct determination of the 
resistances employed in this investigation, owing to delay in delivery, it was originally 
ordered with this object, and the preliminary experiments which were made by Professors 
Ayrton and Jones in testing the apparatus before it was sent out, are of so great value 
as bearing on the absolute value of the ohm that they cannot be passed over without 
mention. The null method of Lorenz, in which a resistance is directly determined in 
terms of the speed of rotation of a disc spinning in the field of a co-axial coil of known 
dimensions, is generally admitted to be the most accurate for the absolute measure¬ 
ment of resistance. The McGill College apparatus was constructed by Messrs. 
Nalder Bros, to my order, under the direct supervision of Professor Yiriamu 
Jones, and embodied all the improvements introduced into the method by himself 
and by Lord Bayleigh. The most important new feature of the design was the 
winding of the coil on a heavy cylinder of marble, instead of metal as employed in 
Professor Jones’ original apparatus. This material possesses the advantage of high 
insulating properties, great rigidity, and small thermal expansion. The employment 
of a marble cylinder made it possible to wind the coil with uncovered wire with the 
object of obtaining the most exact measurement of the dimensions, but on account 
of some difficulties of insulation, the original winding of bare wire was eventually 
replaced by one of silk-covered wire coated with paraffin and shellac varnish. 
The results of the tests made by Professors Ayrton and Jones with this coil, at 
the laboratory of the Central Institution, during 1896 and 1897, have been published 
in the Reports of the British Association for 1897 and 1898, and in the ‘ Electrician.’ 
They give a value for the Board of Trade standard ohm nearly 3 parts in 10,000 
