1881.] Determination of the Ohm in Absolute Measure. 135 



ciable error was introduced by a very slight warping of the wood, and 

 the scales were fonnd to be very accurately divided, but the small 

 errors existing were corrected ; small corrections had also to be 

 introduced, which are due to the imperfect joining of the different 

 pieces. The combined correction never amounted to more than *15 of 

 a division. Each division, as has already been stated, being very 

 nearly equal to 1 millim. 



It has already been noticed that the normal to the mirror pointed 

 slightly downwards. The correction due to this want of adjustment 

 seems to have been generally neglected, yet it is not altogether un- 

 important. If p is the vertical distance between the centre of the 

 objective and that point of the scale where it is cut by the normal 

 to the mirror ; also if a. is the inclination between the normal to the 

 mirror and the horizontal, the correction to be applied to a deflection d 

 is dpa/T), where D is the distance of the mirror from the scale. In 

 our experiments the correction amounted to d X 0'00014, although the 

 angle between the normal and the horizontal was only about 14 minutes 

 of arc. The correction is positive only if the normal lies between the 

 horizontal through the mirror and the line joining the mirror to the 

 cross wires of the telescope.. 



Correction for Temperature. — We have now to discuss a series of cor- 

 rections which have to be applied in order to make a comparison of the 

 results obtained in different spinnings possible. It has already been 

 noticed that four spinnings at the same speed were always taken into 

 one set. The comparison of resistance at the beginning and end of each 

 set showed that during the time of spinning the temperature had 

 altered ; before combining the mean within each set we had, there- 

 fore, to correct for the change of temperature. We endeavoured to 

 keep the room as much as possible at a constant temperature during 

 the experiments ; the lamps used were always lighted nearly two 

 hours before beginning, but, in spite of all precautions, the tempera- 

 ture always rose after we had entered the room and begun to work. 

 The thermometer rose at first pretty rapidly through about 1 degree, 

 and then rose slowly until at the end of the evening it stood generally 

 nearly 2 degrees higher than at first. When the first set of spinnings 

 commenced, the rapid rise, as shown by the thermometer in the room, 

 had already subsided; but, as was to be expected, the temperature of 

 the coil was lagging somewhat behind that of the room, and its re- 

 sistance still rose appreciably. Thus, during the first night, the resist- 

 ance of the copper coil rose almost *4 per cent, during the course of the 

 first set of four spinnings. If the curve of temperature of the coil is 

 known, there is of course no difficulty in applying the proper cor- 

 rection. This curve can be obtained approximately by plotting down 

 the measured resistances as ordinates with the time as abscissae. This 

 was done for all observations made on December 2 ; but during the 



VOL. xxx IT. L 



