A.l.s. to Mr Gavey from W H Preece, referring to problems in understanding Callway's duplex experiments (see 020/254). With reply from Mr Gavey, explaining the system; reply from W H Preece suggesting improvements; reply from Mr Gavey referring to the use of a single-current key in the apparatus; [manuscript copy] of note to Mr Callway from W H Preece, congratulating him on his duplex experiments; reply from Mr Callway, written at Newport, giving credit for the successful method to J Belgion (clerk).