262 REPORT OF STATE BOARD OP HORTICULTURE. 



with the mercury. The thermometer can be tested at any time by compar- 

 ing it with an accurate Fahrenheit thermometer. It is made large in oi'der 

 to give free action to the mercury around the wire in the tube. 



TO PUT THE APPARATUS IN PLACE. 



To prepare the battery, place the copper and zinc in a glass jar as in the 

 cut, and put copper sulphate crystals ( blue vitrol i in the jar to the depth of 

 an inch. Fill the jar with water, being sure to cover the zinc and connect 

 the wire from the copper to the zinc. At the end of twenty-four hours the 

 batteiy will be ready for use. The battery can be prepared for immediate 

 use by filling it to the zinc with water and then covering the zinc with the 

 solution, surrounding the zinc of a similar battery, which is in good running 

 condition. 



The thermometer should be freely exposed not more than two or three 

 feet 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 instru- 

 ment is to give warning, usually 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 pole of the battery, another wire connects the mercury in the 

 bulb 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. It is well to test the connections by seeing that the bell rings 

 when the thermometer circuit is broken. 



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 furnishes a continuous current and will keep the 

 armature in this position until the circuit is broken. When the circuit is 

 broken by the mercury falling below the lower point of the wire in the ther- 

 mometer the armature will be drawn by the spring against the screw S, 

 which puts the bell in a circuit with the battery. This condition is shown 

 in the cut. The wires used in connecting up the aparatus should be insulated 

 by fastening them to wooden supports. It should be noticed that the bell 

 will give warning of the breaking of one of the wires leading to the ther- 

 mometer. In case the thermometer is more than one hundred and fifty feet 

 from the battery, two cells should be connected in series, i. e., the copper 

 of one cell connected with the zinc of the other. In any case where one cell 

 is not sti'ong enough, two in series should be used. 



Since the instrument can be set for any temperature, it can be put to prac- 

 tical use in the greenhouses, incubators, and in any place where a warning of 

 low temperature is desired. 



