TIMERS, COUNTERS AND RATE MEASUREMENT 



the Trochotron and the PhiHps El T; they have the advantage of speed, 

 in that count rates of the order of 200,000 per second are possible. The 

 fastest glow tubes can count at about a tenth of this rate. 



For biological work the glow-discharge types are usually fast enough 

 and it is not proposed to consider cathode ray counters further. As examples 

 of the former, the Ericcson Dekatrons type GCIO B and GCIO D will be 

 briefly discussed; the only operational difference between these two tubes 

 is their upper limiting counting speeds, 4,000 counts per second with the 

 former, 20,000 counts per second with the latter. 



Anode 



First guides 

 Second guides 



i 



Cathode 



Output 

 cathode 



Figure 41.12 



The GC 10 B — The GC 10 B comprises a central disc-shaped anode 

 surrounded peripherally by 30 equally spaced rod-like electrodes. Electrodes 

 1-4-7-10 ... 25 are connected together internally and brought to an 

 external connection as 'cathodes'. Electrode 28 is brought out separately 



HT+ 



Figure 41.13 



as the 'output' or 'carry' cathode. Electrodes 2-5-8 ... 29 are connected 

 together and brought out as 'first guides' and similarly 3-6-9 ... 30 as 

 'second guides'. The gas filling is neon. The circuit diagram is as in Figure 

 41.12. Consider now the circuit in Figure 41.13. On applying the HT the 

 glow strikes between the anode and one of the cathodes, the «th say. There 

 is no possibility of the glow bridging the anode and one of the guides 

 because the latter are biased such that the potential difference is insufficient. 

 However, if a negative pulse of sufficient magnitude be applied via a 



650 



