Determination of the Ohm. 419 



a ship's chronometer of Brocking ? s, which was rated by 

 observation. 



The solenoid is wound round upon a wooden block re- 

 peatedly coated with glue, such as are used in orchestrions. 

 It was made in the year 1868, and was again planed down 

 and polished on the lathe in May of last year. Its dia- 

 meter was determined in three different ways, which were so 

 chosen that it was possible at the same time to ascertain that 

 the section of the drum sufficiently approximated to a circle, 

 and that the whole block was a cylinder. They were : — 



1. Every six diameters of the same section were compared, 

 by means of a micrometer-screw at thirteen equidistant points 

 along the length of the block, with a glass rod measuring 

 23-3264 centim. in length*. 



The diameters found were: — 



Lowest value. Highest value. Mean, 



centim. centim. centim. 



23-3193 23-3286 23-3248 



2. The circumferences were measured by means of strips 

 of paper at thirteen equidistant points, whence the calculated 

 diameters were: — 



Lowest value. 



Highest value. 



Mean. 



centim. 



centim. 



centim. 



23-3186 



23-3252 



23-3229 



3. The diameter was reckoned from the length of the wire 

 wound round it. During the winding the thickness of the 

 wire was at the same time measured at 332 points by means 

 of a microscope with ocular micrometer, and found to be 

 0*0472 centim. The diameter of the block was found to be 



D = 23*3262 centim. 



The measurements in No. 2 were repeated after the winding, 

 D = 23*3190 centim., and were carried out again after each 

 determination, both with strips of paper and a steel-band mea- 

 sure, D = 23*3194 centim. and 23*3204 centim. As the mean 

 of all the measurements, we get for radius R the formula (2), 



R= 11-6846 centim. 



The isolation of the wire coils was tested by means of 

 Hughe's induction-scale f. 



The number of convolutions was 2864 ; they covered the 

 drum to a length of 135'125 centim. 



* The length of this glass rod was determined at the Normal-Standards 

 Commission in Berlin. 



t Cf. Kayleigh, Phil. Trans. 1884, vol. clxxv. p. 419, " On the Electro- 

 chemical Equivalent of Silver." 



