420 Prof. F. Himstedt on the 



We get then 



K __ 2864 



135-125 



The induction-coil is also wound round wood, and consists 

 of 3848 convolutions in 15 sections, which can be combined 

 at pleasure. The width of the bobbin was 4*01 centim. The 

 mean radius of each section was reckoned from the circum- 

 ferences of the separate layers of wires, which had been 

 measured by means of strips of paper. To test the isolation 

 of the coils, and to possess a control over the paper-measure- 

 ments, the mean radius of each section of the wider one was 

 determined by Von Bosscha's method *. As, however, the 

 dimensions of the bobbin are only expressed in the term 

 2a of formula (2), a mistake of one per cent, would only 

 falsify the value of r by at most 0*0003. 



The resistance r, which has to be determined in absolute 

 measurement, amounted either to 1 or -Jr, or 2 Siemens units, 

 and was composed of 2 Siemens units (by Siemen and 

 Halske), nos. 3618 and 3619, which were used either singly 

 or parallel, or behind one another. These were made fast in 

 metal cases, which were filled with oil, and stood in a large 

 bath of the same. Both before and after the experiments, 

 both standards were compared with a third unit, and with 

 each other. 



The thermometers were divided into one-tenths of a degree, 

 and were three times compared carefully with the standard 

 thermometers of the Physical Institute. 



The interrupter was driven by a toothed wheel f. For 

 determining n, the number of interruptions, a counter was 

 made fast to the axis of rotation, by means of which ^ of a 

 revolution could be directly read off. 



At least 700 revolutions were counted during each experi- 

 ment, so that the necessary approximation of 0'01 per cent, 

 was easily attained. The apparatus worked very evenly, so 

 that the divergences of the galvanometer were constant 

 throughout. 



This appears to me an advantage that this interruptor 

 possesses over that used by Herr Roiti J ; and I believe that 

 the discrepancies between his separate observations are due, 

 if not entirely, at least in a great measure, to the unequal 

 action of the u Schmidt " water-motor used by him. 



* Wiedemann, Elec. iii. p. 213. 



t F. Himstedt, " Zwei Verschiedene Formen eines selbstthatigen Dis- 

 junctors," Wied. Ann. vol. xxii. p. 270 (1884). 

 \ Cap. I. page 11. 



