PHYSICS. 481 



Mascart, De Nerville, and Benoit bave made a deteriniuatiou of the 

 length of the mercury column required by the legal oliui, using for this 

 purpose the methods of Kirchholi" and Weber. They conclude as fol- 

 lows : " The experiment appears to us to show that the value of the 

 ohm is surely comprised between 106.2 and 106.4™'; we may consider 

 the value 106.3 as exact, at least to yJ^^. part." (0. B., April, 1884, 

 XCViii. 1034; J. Fhys., June, 1884, II, in, 230.) 



Benoit has constructed for the French Government four mercury 

 standards corresponding to the ohm of the conference. These are 

 straight glass tubes whose caliber is nearly 1""". The resistances as 

 measured were 1.000017, 0.999996, 0.999960, and 1.000003 ohms, or 

 0.999994 as a mean. (0. R., xcix, 864, November, 1884.) 



The committee of the British Association has announced that arrange- 

 ments have been completed for testing resistance coils at the Cavendish 

 Laboratory, and issuing certificates of their value. They assume the 

 B. A. unit to be 0.9867 of a standard ohm. {Nature, xxix, 465, March, 

 1884.) 



Edelmann has modified the quadrant electrometer of Thomson by 

 making the quadrants cylindrical, formed by slitting a metal tube into 

 four parts b,y four equidistant cuts parallel to the axis. The needle 

 consists of two portions of metal cut from a cylinder, united above and 

 below and hung by a single fiber, directive force being given by a small 

 attached magnetic ueedle. {Nature, January, 1884, xxix, 239.) 



Chervet has described a new form of capillary electrometer with hor- 

 izontal tube, in which this tube is slightly conical, and which is empir- 

 ically graduated. With a lens magnifying six times, a difference of po- 

 tential of 0.00001 DauielLcell may be observed. {J. Fhys., June, 1884, II, 

 III, 258.) 



Garbe has laid down the two following laws in relation to the cap- 

 illary electrometer of Lippmann : (1) The capillary constant of mer- 

 cury is greatest when the electrical difference at the meniscus is nil, 

 and as a rule its value is independent of the sign of this ditference. 

 (2) The electrical capacity at a constant surface of an electrode plunged 

 in a liquid is purely a function of the electrical difference, independent of 

 the sign of that difference, and is least when that difference is nil. {Na- 

 ture, October, 1884, xxx, 568.) 



Lippmann has contrived a mercury galvanometer based on the ac- 

 tion of magnets on currents. A mercury manometer in the form of a 

 U-tube is placed between the poles of a permanent magnet, so that 

 these poles are on the right and left side of the horizontal portion. 

 The current to be measured is made to traverse this horizontal portion 

 in a vertical direction ; i. e., perpendicular to the axis of the tube. A 

 difference of level is thereby i)roduced in the manometer columns pro- 

 portional to the strength of the current. In one of the author's instru- 

 ments this difference is equal to 23"'" for 1 ampere ; in another to 58""" for 

 1 ampere. The apparatus is reversible; by moving the mercury me- 

 S. Mis. 33 31 



