COUNTERS 



This seemingly complicated behaviour can be achieved quite simply v^'ith 

 the help of an extra pair of diodes in a circuit due to Rotblat, Sayle and 

 Thomas^. The relevant fragment of circuitry is shown in Figure 41.11. 

 When the fourth circuit is 'off', the right-hand triode is conducting, Dg is 

 cut off and pulses are transferred from the fourth to the second Eccles- 

 Jordan circuit via D^. When the fourth circuit is 'on', the right-hand anode 

 rises to HT+, rendering D^ conductive. At the same time, the left-hand 

 anode is now negative to HT+, D^ conducts and cuts off D^. In this manner 

 pulses arising from Eccles-Jordan No. 1 are directed to Eccles-Jordan No. 4 

 and blocked from Eccles-Jordan No. 2 (for further details the original 



From first Eccles-Jordan 



Fourth Eccles-Jordan 

 circuit 



Figure 41.11 



article should be consulted). Scale of ten counters can be connected in 

 chains to form scales of 100, or 1,000 and so on. A cheaper kind of scaler, 

 using Eccles-Jordan circuits but relying on capacitors instead of diodes 

 for coupling, has been discussed by Gottlieb^. It seems to the author that 

 circuits of this kind might be rather more difficult to get going than the 

 diode-coupled variety. Eccles-Jordan design procedure has been considered 

 by Taubio. 



Scalers of the Eccles-Jordan type are reliable and widely used, but they 

 contain a large number of valves, and special decade counting tubes have 

 been developed to effect simplification and gain economy here. These 

 have been reviewed by Kandiah^^. Briefly, they fall into two categories: 



(1) Glow discharge types in which a glow occupies one of ten positions, 

 circularly arranged, in a soft valve. Each input pulse moves the glow on 

 to the next station, and the count is read off" by observing the position of 

 the glow against an external annular numbered escutcheon. At the com- 

 pletion of each complete circuit of the glow the tube delivers a 'carry' pulse 

 which may be used to operate further tubes. 



(2) Cathode ray types, in which a cathode ray beam in a hard valve is 

 arranged to occupy one of ten stable positions. At each position the electron 

 beam bombards an appropriate numeral, made of fluorescent paint, causing 

 it to glow and enabling the count to be read off. The beam is transferred 

 to the next position by setting up electric fields within the tube with suitable 

 electrodes. 



Glow discharge decade counter tubes are known variously as Dekatrons 

 or Nomotrons, according to the manufacturer. Cathode ray types include 



649 



