value entered into the second register may be zero so that the value in the second register will represent elapsed time and the value entered in the first register will then represent the time delay before firing of the charge the control means 16 includes a liquid crystal or other display which can be seen through the transparent panel of the control means housing and in which the contents of either register of the timer 31 can be displayed the control means 16 also includes an override switch 34 which is normally held in a first condition by a removable pin not shown accessible at the front of the device in its first condition the override switch provides a connection between on the one hand the second switching means 24 and on the other hand the timer 31 and the signalling means 20 in its second condition which is represented in fig3 the override switch provides a connection between the timer 25 and the second switching means so that the timer 25 is able to set both the first and second switching means into a conducting condition the override switch 34 is changed automatically into its second condition when its pin is withdrawn and is not then accessible so that it cannot be returned to its first condition the first and second switching means 23 and 24 are preferably semiconductor devices for example thyristors as shown in fig4 the device illustrated in the drawings would normally be manufactured without the charge 13 and detonator 19 the magnets 12 float 15 and control means 16 including the.