the closing window glass and will cause a force f to be applied to the outwardly facing surface 78 of the material of the side wall 50 a of the channel 50 this force will be transmitted by the material of the channel to the ridge 71 causing the base layer 16 to flex so that the conductive strip 18 moves into electrical contact with the conductive strip 14 an electrically detectable control signal will therefore be produced which can be used to cause immediate de energisation of the window glass driving motor advantageously followed by reversal of the motor to lower the window glass away from the obstruction the construction of the sensor 5 in the opposite side wall 50 b of the channel 50 is the same as shown in fig5 the base of the channel 50 is provided with two longitudinally extending chambers 72 to increase the resilience of the side walls of the channel this additional resilience ensures that only a low reactive force is applied to the obstruction by the window glass during the very short period of time in which it may continue to rise after the sensor 5 has produced the control signal clearly the resilience of the side wall must not be so great as to reduce the sensitivity of the sensor the hollow chambers 72 may be omitted the arrangement shown in fig6 is advantageous in that it will not only detect a force f applied to a small part of the total area of the surface 78 e g insertion of a human finger into the window.