In connection with the testing of wireless receivers, information was desired concerning the field strength obtained from 5XX in Cornwall, this district being known to be poor from the point of view of wireless reception.An investigation of the field strength was therefore carried out with the aid of portable equipment. This particular apparatus was simpler than that usually employed but appears from the consistency of the results to be reliable.When the experiments were conducted, a curious effect was brought to light which appeared to be a radio shadow caused by the masts of the beam station at Bodmin. This gives rise to an interesting distribution of the contours in the Truro district.The general order of field strength appears to coincide with theory up to this point, if due allowance is made for attenuation, but there is a rapid falling-off towards the extreme west of Cornwall.