298 M. F. Kohlrausch on the Absolute Value of the 



weighing about 2 grammes. But, in addition, this little sphere 

 was purposely weakly magnetized and did not possess more 

 magnetism than can be imparted to a sewing-needle of the mass 

 °/to of a gramme, as I have assured myself by experiment*. It 

 may serve further to illustrate the figures to state that a 

 small elongated iron rod of 10 grammes, held in the direction 

 of dip, would acquire the above degree of magnetization through 

 the inductive action of the earth's magnetism. A single silk 

 fibre, 2 metres long, was required as the suspending-thread for 

 the steel sphere, in order to reduce the torsion to the small 

 amount prescribed by the varying elasticity and elastic reaction 

 of the silk. Now let a magnetized sewing- needle of the very 

 smallest size be conceived of as fastened by a connecting-piece 

 about \ metre in length to a mirror of 30 millims. diameter, 

 and therefore (according to the drawing) offering a surface of 

 about 14 square centims. for the action of air-currents, which, 

 even in a well -closed case, cannot be entirely avoided ; further, 

 let the whole mass have such a moment of inertia that its time 

 of vibration is ten seconds (p. 173), while that of the sewing- 

 needle by itself would have been about two thirds of a second, 

 and we have an idea of the essential part, of the magnetometer 

 acted on by the weak currents of the multiplier, an error of two 

 minutes of arc in the determination of whose position would 

 cause an error of 1 per cent, in the result. Moreover the large 

 multiplier was revolving in the immediate neighbourhood of this 

 magnetometer with a velocity sometimes as high as four revo- 

 lutions in a second. 



It appears to be a defect in the otherwise very complete reports, 

 that, so far as I am aware, there has been no publication of the 

 full details of a series of observations such as would afford 

 ground for accepting or rejecting the objection that has been 

 urged above. It is mentioned (page 1 74) that individual parts 

 of the lengthy series of experiments were rejected before the 

 calculation was made, on account of their want of agreement with 

 others f ; whence it appears that considerable unexplained irregu- 



* The steel sphere deflected a compass-needle at a distance of 156-6 mm. 

 through an angle =tan~ x 0-0078=27' (Brit. Assoc. Rep. 1863, p. 172). 

 Hence, taking the horizontal intensity=l "76, we get for the magnetic 

 moment, M=£ . 176 . 156'6 3 . 0-0078=26000. But since 1 milligr. of steel 

 will assume a maximum of permanent magnetism amounting to about 1 000 

 units (comp. also Schneebeli, Progr. des Z'urch. Polyt. 1871-72, Pogg. 

 Annal. Erg'anzungsbd. vi. p. 150), the above degree of magnetization might 

 be given to a thin rod of 26 milligr. 



[t The German words are wegen Nicht- Uebereinstimmung mit anderen, 

 which are not equivalent to those used by the Brit. Assoc. Committee at 

 the place referred to, namely, " any portion of an experiment accidentally 

 vitiated could be rejected by itself." — Transl.] 



