38 



Mr. W. L. Balls. 



[Dec. 22, 



-J L. 



The contact being thus broken, the arm swung up into equilibrium, the 

 gas-holder dropped back to temporary equilibrium in the beaker, and a fresh 

 stroke of the pump commenced. A slight alteration in pressure is involved 

 by the necessity for depressing the contact arm, but this is reduced to a 

 negligible amount by making the contact arm as light as possible. 



Lastly, the connections were arranged as follows : from the battery to the 

 solenoid, then to the metal balance-post, and 

 through its pivot down the contact-arm to its 

 platinum tip. When the circuit had been 

 completed as described above, the current 

 passed into the corresponding platinum wire, 

 which it met at right angles on the insulated 

 end of the bracket, and from there passed by 

 insulated wires into the holding-down magnet, 

 down the side of . the balance pillar to the 

 relay (R) and back to the battery. 



(b) The Relay. — This was simply an electro- 

 magnet, wound with thick wire, and attracting 

 an armature against a spring. When the 

 circuit was closed, and the armature drawn 

 down to the magnet, a contact (Cr.) was made 

 by the armature, which- closed the separate 

 circuit of the telegraph recorder. When short 

 lengths of telegraph wire are employed, as 

 when working indoors, the relay can be 

 omitted. 



(c) The Battery. — This consisted of three 

 " W.-O." medium cells, which worked the 

 appliance almost continuously for over two 

 months. 



(d) The Case. — The box containing pump, 

 relay, and battery, was 30 cm. high, and 20 cm. 

 in width and depth. It was provided with a 

 dust-proof door in front, a handle for transport, 



and bore on its top two binding-screws (t, t) from the relay, for the telegraph 

 wire, together with a glass nozzle (p) leading from the exit tube of the pump. 

 When placed on the ground beneath the plant to be examined, care was taken 

 to level it approximately, so as to avoid friction of the core on the glass 

 solenoid-lining. 



(e) The Recording Appliances {Fig. 2). — These were placed in the laboratory 



SI 



JJ_ 



J 



