  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 $\sigma =0.27\pm 0.05$. Further, there is agreement
with the ZK prediction for the overall normalisation.
