NEGRETTI AND ZAMBRA, HOLBOEN VIADUCT, E.G. 16 A 



Negretti and Zambra's Self -compensating Standard Barometer 



consists of the usual form of standard instrument, but attached to the vernier 

 is a double rack moved by one pinion, so that when adjusting the vernier in 

 one position, the second rack moves in the opposite direction, carrying along 

 with it a plunger (the exact size of the internal diameter of the tube) dipping 

 in the cistern, so that whatever displacement has taken place in the cistern, 

 owing to the rise or fall of the mercury, it is exactly compensated by the 

 plunger being more or less immersed in the mercury, consequently no capacity 

 correction is required. Price, 18 18 



Standard Barometer, with Electrical Adjustment. This barometer 

 consists of an upright glass tube dipping into a glass cistern of mercury, so 

 contrived, that an up-and-down movement, by means of a screw, can be imparted 

 to it. Through the top of the tube a piece of platinum wire is passed and her. 

 metically sealed. The cistern also has a metallic connection, so that by means 

 of copper wires (in the back of the frame) a galvanic circuit is established ; 

 another connection also exists by means of a metallic point dipping into the 

 cistern. The circuit, however, can be cut off from this by means of a switch 

 placed about midway up the frame. On one side of the tube is placed a scale 

 of inches ; with a small circular vernier, divided into 100 parts, connected with 

 the dipping point, and working at right angles with this scale. 



Eor taking an observation, a galvanic battery is connected by two binding 

 screws at the bottom of the frame. The switch is turned upwards, thereby 

 disconnecting the dipping point ; the cistern is then screwed up, so that the 

 mercury in the tube is brought into contact with the platinum wire at the top ; 

 the instant this is effected a magnetic needle arranged as a galvanometer on the 

 barometer board will be deflected. The switch is now turned down ; by so 

 doing the connection with the upper platinum wire is cut off, and established 

 between the dipping point carrying the circular vernier and the bottom of the 

 cistern ; the point is now screwed by means of the milled head until the needle 

 is again deflected, and the line on the vernier cutting the division on the scale 

 is the exact reading of the barometer. Price, 18 18 



The two Barometers above-mentioned were exhibited 1y Negretti and Zambra at the 

 Meeting of the Royal Meteorological Society, March, 1886. 



