METEOROLOGY. 



315 



•Vosts hy means of siiiudi^''os, and (Icscrihos a pioco of cloctrical a])pa- 

 ratiis devised by the author to give warning- of a fall in tempenitme. 



"The apparatuH consists of a battery, relay coil, alarm hell, and thenuometer. 

 The battery used is the common crow-foot cell used in telegrapliin<f, size 6 in. by 

 S in. 



"The relay C (tig. 4) is composed of two coils. Each coil has an iron core 1^ in. 

 Inng, 5 in. in diameter, and is wound with No. 24 B. and S. double cotton-covered 

 magnetic wire, to a depth of I of an inch. The coils are wound right and left handed 

 and are placed horizontally. 



Z77. 



Cix 



C 



OPWfll 



Fu;. ■!.— Electrical apparatus for frost warning. 



"The armature A consists of an upright piece with a crossbar of soft iron, which 

 is so held that it is attracted by the iron cores of the coils when magnetized. An 

 adjustable spring holds the armature A against the screw S when no current is flowing 

 through the coil C. . . . 



"The thermometer consists of a glass stem 8 or 10 in. long, with an internal 

 diameter of approximately jV of an inch (0.1 cm.), attached to a bulb which has a 

 diameter of 1 in. (2.5 cm.). The bulb and 2 or 3 ins. of the lower part of the 

 tube are filled with mercury, and the tube is graduated for every 10 degrees from 

 30° F. to 100° F. Electrical connection with the mercury is made by a platinum 

 wire blown in the glass. On the top of the stem is a brass cap with a No. 30 B. and 

 S. bare copper wire passing through it and niaking contact with the mercury. The 

 thermometer can be tested at any time by comparing it with an accurate Fahrenheit 

 thermometer. It is made large in order to give free action to the mercury around 

 the wire in the tube. . . . 



"The thermometer should be freely exposed, not more than 2 or 3 ft. above the 

 ground, in that portion of the orchard most liable to frost, usually the lower ground. 



"The lower point of the copper wire in the tube of the thermometer should be 

 set opposite the number indicating the temperature for which the instrument is to 

 give warning, iisually 40 to 45° F. A copper wire, No. 16 B. and S., connects the 

 wire in the stem of the thermometer to one post of the bell and to one jwle of the 

 battery, another wire connects the mercury in the bull) with the proper binding post 

 of the coil, and a third wire is run from the armature of the relay to the remaining 

 binding post of the bell. The remaining pole of the battery is then (connected to 

 the properly marked post of the coil. . . . 



"The action of the instrument is very simple. When the wire in the stem of the 

 thermometer is in contact with the mercury a current of electricity will pass through 

 the relay coils, causing the armature to be attached to the iron cores. The battery 



