Intelligence and Miscellaneous Articles. 227 



The differences are not, in general, greater than those existing 

 between the numbers calculated by Dulong and Petit from their 

 formula and the observed numbers. 



From the whole of these facts it appears to me to follow that, 

 between 0° and 1775°, the law of radiation can be represented by 

 the formula 



Comptes Rendus de VAcacUmie des Sciences, t. xcii. p. 1204. 



OK WHBATSTONE S BEIDGE. BY K. F. SLOTTE. 



The length of the platinum wire belonging to this apparatus, 

 which cannot be exactly determined by measurement, can, it is well 

 known, be ascertained indirectly by comparing and exchanging 

 resistances*. The following procedure is a modification of this 

 method which may not be without advantages. 



Let s be the length of the wire, a and b that of its two divisions 

 when two resistances u\ and w 2 are inserted and the galvanometer 

 shows no current. Then is 



w v _a_ s + a—b s + t? t ... 



iv 2 b s— (« — b) s—d x 



in which a— 6 is put = d v If now the resistances iv x and iv 2 be 

 exchanged and the movable contact shifted till again no current 

 passes through the galvanometer, this displacement, taken as posi- 

 tive or negative according to whether a is more or less than 6, is 

 equal to d L , which quantity can be directly determined by reading 

 it off upon the scale of the apparatus. 



If in the same manner w 2 be compared with a third resistance 

 iv 3 , and, again, iv 3 with w v and if the displacements corresponding 

 to d x be denoted respectively d 2 and d 3 , then we have 



vr _ s + d 2 



^-JI^ W 



^ = £±^3 (3 ) 



From (1), (2), and (3) we get 



( s+ d x )(s+d 2 )(s+d 3 ) 

 (s-d^s-djis-dj 

 and finally, by solving the last equation, 



V 



d,d n d» /q 



d x + d 2 + d 3 



Three determinations effected by this method, in which w 2 was 

 chosen approximately equal to V w x w 3 , gave for s the values UlS'lL 

 1118-75, 1118-6 millim. — "Wiedemann's Annalen, 1882, no. l,vol.xv. 

 p. 176. 



* See G. Wiedemann, Galvanismus, [2] i. p. 254. 



