  It has been argued by Zurek and Kibble that the likelihood of producing
defects in a continuous phase transition depends in a characteristic way on the
quench rate. In this paper we discuss our experiment for measuring the
Zurek-Kibble scaling exponent sigma for the production of fluxons in annular
symmetric Josephson Tunnel Junctions. The predicted exponent is sigma = 0.25,
and we find sigma = 0.27 +/- 0.05. Further, there is agreement with the ZK
prediction for the overall normalisation.
