1911.] 



The Stomatograph. 



37 



This rod was then drawn down into the solenoid, raising the gas-holder 

 rapidly until a fresh charge of air had been taken in, when the contact was 

 broken. The figure shows the position of the pump at a fraction of a second 

 before the admission of a fresh charge, while the solenoid circuit is still 

 intact. 



The use of a solenoid and core in this way has several advantages, the 

 chief of which is that its lifting power is greatest when most required, 

 namely, at the stage shown in the figure. The only other difficulty then left 

 was to provide a prolonged contact, which should be broken when the fresh 

 charge had entered the gas-holder, and not until then. This was effected by 

 a separate light swinging arm (Ca) behind the main balance, but on the same 

 pivot, shown also in plan in fig. 1b. 



Fig. 1] 



This arm was counterpoised by a sliding weight so as to rest normally at 

 an inclination of 30° to the horizontal (Ca' , Ca'), the elevated end being 

 that which bore a platinum wire contact. As the gas-holder side of the 

 balance descended, a projecting rod (a) which it bore, pressed upon this 

 elevated end, and carried it down also, until the platinum wire came into 

 contact with the platinum-tipped surface of a fixed bracket (Br) projecting 

 from the side of the balance-pillar but insulated from the latter. This 

 contact completed the circuit through the solenoid and caused the gas-holder 

 to begin to rise ; in order to prevent the contact beam from swinging up 

 immediately from the bracket, and so breaking the contact, a small electro- 

 magnet (M) was placed under it, near its pivot, and included in the same 

 circuit ; this magnet was wound so as to be only sufficiently powerful to 

 keep the contact arm in contact, by means of a ferro-type armature (A). 



Contact was thus held until the new charge of air bubbled into the gas- 

 holder, when the sudden diminution in weight of the latter allowed the core 

 to jump further into the solenoid. At this moment a similar projecting 

 rod (b) on the core side of the balance-beam engaged with the other end of 

 the contact arm and knocked it off from the holding-down magnet. 



