460 



NEGRETTI AND ZAMBRA, HOLBORN VIADUCT, E.G., 



(SI (51 



METALLIC THERMOMETER'S ELECTRICAL ALARM. 



2800 The engraving (fig. 2800) shows the general arrangement of the Metallic 

 Alarm Thermometer, designed for regulating the temperature in incubating houses- 

 hot-houses, green-houses, drying stores or rooms, hospital or prison wards, &c- 

 They are also valuable as a fire-alarm, either in large buildings or warehouses or on 

 board of ship, and they will notify any change of temperature taking place where 

 the dial A is placed to almost any distance that may be desired. 



The construction and action of the Alarm ia as follows. At the back of the 

 dial A is fixed a coiled metal spring, in such a manner that it is very sensitive to any 

 changes of temperature, it being lengthened by increased heat, and shortened by 

 cold : these movements are shown by an index in front of the dial A, which has 

 upon it a scale graduated into degrees corresponding to a Thermometer either of 

 Fahrenheit's, Centigrade, or Reaumur, as may be desired. On the glass cover of 

 the dial are mounted two movable indices, in such a way that they can be turned 

 round to any two points of the divided circle, and there fixed ; these two hands 

 represent the highest and lowest temperature required to be notified, the central or 

 moving hand being in connection, by means of a metallic wire, with one pole of a 

 galvanic battery B, and the other two indices being connected by another wire to 

 the other pole of the battery c. 



Now, should the temperature of the atmosphere surrounding the dial alter, the 

 central hand will be moved either to the right or left, and the moment it touches 

 either of the outer hands, contact will be made, and the galvanic circuit completed, 

 causing the Bell to ring, and give notice to the watchman that the temperature of 

 the building or vessel has undergone some alteration and requires attention. 



The dial A may be fixed at any distance from the Alarm Bell B, dependent only 

 on the amount of battery power used, one battery being sufficient to work the 

 apparatus between any two portions of an ordinary sized building. 



Price of Dial, Alarm Bell, and 1 Battery (tig. 2800) . .550 



