1881.] Determination of the Ohm in Absolute Measure. 121 



a still further reduction of nearly two parts in a thousand must be 

 made in the number which expresses the ohm in absolute measure, and 

 we should get — 



1 ohm=-9910 earthc ' uadrant , 

 second 



coinciding practically with the value obtained by Rowland from his 

 own experiments. 



In the course of our experiments various doubts suggested them- 

 selves, and were subjected to examination. It may be well to say a 

 few words about some of these, though the results are for the most 

 part negative. 



The energy of the currents circulating in the coil is expended in 

 heating the copper, and a rise of temperature affects the resistance. 

 Calculation shows that the disturbance from this cause is utterly 

 insensible. If at the highest speeds of rotation all the heat were 

 retained, the rise of temperature would be only at the rate of 3'2 X 

 10~ 8 ° C. per second. 



Much more heating may be looked for during the operation of taking 

 the resistance. Under the actual circumstances a rise of resistance of 

 about one part in 30,000 might be expected as the effect of the battery 

 current in one minute. The aggregate duration of the battery contact 

 in each of the resistance measurements was probably less than a 

 minute. 



Another question related to the possible effect of a want of rigidity 

 in the magnetism of the needle. It is known that galvanometers will 

 sometimes, when it is certain that there is no average current passing 

 through the coils, show a powerful effect as a consequence of fluctuat- 

 ing magnetism corresponding to the fluctuating magnetic field. In 

 the present experiment the magnetic field is fluctuating, and the 

 magnet is expected to integrate the effect as if its own magnetism 

 were constant. It is unlikely that any appreciable error arises in this 

 way, as I find by calculation that a theoretically soft iron needle would 

 point in the same direction as a theoretically hard needle when placed 

 at the centre of the revolving coil. 



From the details given the reader will be in a position to judge for 

 himself as to the accuracy of our experiments. If, as we believe, the 

 principal error to be feared is in the measurement of the coil, there is 

 little to be gained by further experimenting with the present apparatus. 

 Accordingly a new apparatus has been ordered, from which superior 

 results may be expected. In designing this several questions pre- 

 sented themselves for solution. 



All corrections being omitted, the effect — 



tan oc 



R 



k 2 



