GLASS MICROCAPILLARY ELECTRODES 



A glass tube is passed through a platinum loop 3 mm in diameter and 

 length and is held in a stationary clamp at one end and at the other in a clamp 

 mounted on the plunger. The plunger is supported on ball races and is 

 prevented from rotating. In its traverse it actuates two switches, one 

 causing an increase in the pull of the solenoid and the other turning off 

 the current to the platinum heating loop. 



Once the tube is clamped in position and power supplied the action is 

 automatic. 



I Heater current is turned on. Solenoid exerts small tractive force. 



II Glass softens and extends. 



III (a) At pre-set point of extension heater current turned off. (b) At 

 independently variable point force of solenoid greatly increased. 



IV Solenoid current turned off. 



The construction of this machine has been described in detail^^ and will 

 only be supplemented here with some notes, based largely on experience 

 with a machine constructed in a modified form by R. H. Cook {Plate 35.1) 



(1) Heater element. The clamps supporting the loop on to the heat 

 resistant insulating block should be of minimal size so as to reduce their 

 heat capacity, which otherwise prolongs the heating time. The element 

 being in a vulnerable position, and liable to damage, can have a guard 

 placed close on one side attached to the stationary clamp. This acts as a 

 guide when fitting the tubing to be drawn {Plate 35.2). When required the 

 loop is reformed around a 3 mm rod with pointed ends supported by the 

 clamps. 



(2) It should be possible to control and re-set the various parameters to 

 any determined position, so that systematic trials can be made and different 

 types of electrodes drawn at will. 



(a) The most important variable is the temperature of the heater. The 

 heater current is controlled by a rheostat and measured with a meter. The 

 platinum loop is operated at a red glow temperature when small irregularities 

 can be seen in the detail of the surface of the metal. 



(b) The points in the cycle where heating current is stopped and the final 

 strong pull commenced should be adjustable to fine limits, and in the 

 instrument shown in Plates 35.1 and 35.2 their positions can be noted on 

 mm scales. The contacts controlling solenoid current and heater tempera- 

 ture should perform these functions through intermediate relays. 



(c) The strength of the initial and final pull can be made adjustable. The 

 initial pull should be rather weak, between 10-50 g, increasing to a maximum 

 of over 1 ,700 g, according to Alexander and Nastuk^^. 



(3) When electrodes are drawn, the velocity and mass of the plunger 

 combine to deliver a considerable blow to the instrument which may cause 

 breakage of tips as a result of the vibration. This may be reduced by 

 providing a friction break or cushion of soft plastic material. A new 

 development of the apparatus by B. H. C. Matthews employs the solenoid 

 to produce terminal deceleration as well as the tractive pull. This is done 

 by reducing the length of the solenoid so that as the iron mass of the plunger 

 passes through the direction of the field reverses. To arrest further move- 

 ment a stepped ratchet brake is attached to the end of the plunger which 



540 



