Dr. Matthiessen on the Specific Resistance of Metals. 361 



city of rotation of the edge of the blade nearest to the axis, and 

 for one float 





R = 



= ip,e>Vx -388103 







:±/yo 2 (radius) 4 x '001516, 





M = 



:lp/»Vx 1-320900 







■•ipp* (radius) 5 x -001290. 



Therefore th 



e efficiency at full speed 



Mco~ 



•001516 

 •001290 



v -001516 1 

 X co (rad.) " -001290 X 2 



= •588; 



showing that a screw-surface which has the part nearest to the 

 axis cut away is not so advantageous as a complete screw of the 

 same size which has its axis at a greater distance from the cen- 

 tre of the paddle-wheel. 



In consequence of the proximity of the paddle-wheel to the 

 side of the vessel, it is found that the inner blades do not throw 

 the water under the vessel, but against the side, so that it can- 

 not escape ; hence it has been found necessary to make all the 

 screw-blades on the same wheel of the same kind, and to place 

 them so as to throw the water outwards. As the pressure pro- 

 duced on the water is precisely the same with a left-handed as 

 with a right-handed screw, it is quite clear that the results of the 

 mathematical investigation will be equally trustworthy, whether 

 the corresponding screw-blades be of the same or of opposite 

 kinds. 



LI. On the Specific Resistance of the Metals in terms of the B. A. 

 Unit (1864) of Electric Resistance, together with some Remarks 

 on the so-called Mercury Unit. By A. Matthiessen, F.R.S., 

 Lecturer on Chemistry in St. Mary's Hospital*, 



AS the B. A. unit will doubtless come into general use, it 

 may not be out of place to give the resistance of the metals 

 and some of the alloys in terms of it. Mr. Hockin and myself 

 have determined f, in terms of this unit, the values of the re- 

 sistance of certain lengths and weights of wires of several metals 

 and one alloy ; and when reduced to the same length and weight, 

 namely, a metre long and gramme weight, we found their resist- 

 ances to be, for hard-drawn wires, 



* Communicated by the Author. 



t Not yet published. These experiments were carried out for the Com- 

 mittee appointed by the British Association to report on Electrical Stand- 

 ards, and will appear in their Report of 1864. 



Phil. May. S. 4. Vol. 29. No. 197. May 1865. 2 B 



