( 724 ) 



will influence the result 1 ), the following causes may be mentioned. 

 1. The roughness of tin' wires. Already Mr. Ornstein pointed 

 this out. If' the wire is not entirely smooth, the hydrodyuamical 

 deductions are uncertain and hence also formula (3). In order to 

 ascertain the influence of this roughness I made some experiments 

 with a steel wire that had for a moment been scoured with fine 

 sand-paper in the direction of its length. Macroscopically no result 

 of this manipulation could be discoyered on the wire. Yet the effect 

 proyed considerable, for the following results were found : 



p a 



, 455 0,009 j 



steel wire, diameter 0,5 m.M. 5 J 255 0,011 average 0.011 m.M. 



f 2205 0,014 1 



steel wire, diameter 0,3 m.M. 1255 0.028 



So we find a diminution of about 40% in l ' ,c value of C. After 



haying observed this influence 1 tried to obtain smooth wires, but 



unsuccessfully ; all the wires that were used in the experiments showed 



under the microscope numberless grooves in the direction of their 



length and of a breadth that might be estimated at somewhat less 



than 0.01 mm. 



d 

 Since it is easily deduced trom the calculated values of — that 



the thickness of the layer of water increases with the size of the 

 radius of the wire and since the influence of the roughness of the 

 wire will be smaller with a greater thickness of this layer of water, 

 I have still made some measurements with a thicker steel wire of 

 0,87 mm. diameter and heavier weights. The result was : 



P C C (in C.G.S. units) 



25200 0,00803 

 25200 0,00822 



0,0081 13.8X1U- ,U 



5200 0,00667 11.3 x 10~ 10 



while calculation gives 



d 

 ^ = 0,166, A- 2 = 0,0015, i?=:0,0435 — = 0,00120 C = 21.1 ;< 1 0~ lu 



R 



The agreement is now better indeed ; the value found is half the 



calculated one, while with the thinner steel wires it was slightly 



more than a third. 



*) The values given by F. Kohlrausch (Lehrbuch der praktischen Physik 10 

 Auflage 1905), steel fe = 0.06 to 0.12 and silver A: = 1.01, would give a much 

 better agreement. 



