PHYSICS. 283 



will coutinuoiLsly and by amounts exactly known by a simple reading, 

 until the compensation is exact. The instrument resembles that of Du 

 Bois Keymond, but it uses a capillary electrometer in place of a galva- 

 nometer, and thus, while more delicate and more rapid, polarization is 

 completely avoided. Moreover, the sensitiveness of the method is inde- 

 pendent of the resistance of the cell, batteries of 10,000,000 ohms resist- 

 ance being measured with ease. For weak electromotive forces, the 

 delicacy is one ten-thousandth of a Daniell cell ; for stronger ones, to one 

 two thousandth of a Latimer-Clark cell. The applications of the method 

 to determine the contact-potential are very ingenious.— (J. P/<^6'.,ix,145, 

 May, 1880.) 



Debrun has made a useful modification of Lippmann's capillary elec- 

 ti-ometer. In the original instrument, in a tube a millimeter in diameter, 

 the mercury level changed only 3™'» for a difference of potential of one 

 volt, and to measure small fractions Lippmann used a microscope. In 

 the new instrument this delicacy is secured by making the tube inclined 

 to the vertical, the effect increasing with the angle. A glass tube 

 7™'" in diameter is drawn out so as to give a capillary tube slightly con- 

 ical, and a good millimeter in diameter and 20''"' long. This is bent into 

 a Z-form, the connecting portion being placed horizontally, but fastened 

 to a movable board, so that its inclination may be adjusted. The ends 

 of the tube open into reservoirs. It is filled with mercury and acid- 

 ulated water, as usual, the end of the mercury column being adjusted to 

 occupy three-quarters of the horizontal part. Electrical connections are 

 made with the mercury in each of the reservoirs. The calibration is 

 efiected by using a battery of known potential, increasing the cells.— (J. 

 Phys., ix, 160, May, 1880.) 



NiemoUer has found the telephone capable of determining very quickly 

 and accurately the resistance of licpiids. It is substituted for the gal- 

 vanometer in a Wheatstone's bridge, and an induction current is used. 

 If the resistances to be compared are a large liquid resistance and a 

 Siemens resistance box, so that the electrodynamic constants are very 

 small ; and if also a German-silver or platinum wire be used as measur- 

 ing wire, it is found- that in the position where the galvanometer shows 

 no deflection, the tone in the telephone has a well-marked minimum of 

 intensity. In a liquid resistance of 2,000 units a variation of even four 

 units may be detected remliij. —{Wature, xxi, 309, January, 1880.) 



Trowbridge has studied the conditions which cause the ticking of the 

 time-clock of Harvard College Observatory, transmitted electrically by 

 wire from Cambridge to Boston, to be heard on all the telephone cir- 

 cuits in the neighborhood of the line. He shows from theoretical con- 

 siderations that the usual explanation of induction between the wires 

 is erroneous, and that the effects observed on telephonic circuits, which 

 have usually been attributed to induction, are really due to the earth 

 connections and to imperfect insulation. As the restilt, then, is due to 

 the fact that the wires on which the sound is heard obtain their current 



