, and could fit into a small attache case, although the B Mk II, otherwise known as the B2, which weighed , was required to work over ranges greater than about . Operating procedures were insecure at first. Operators were forced to transmit verbose messages on fixed frequencies and at fixed times and intervals. This allowed German direction finding teams time to triangulate their positions. After several operators were captured or killed, procedures were made more flexible and secure. The SOE wireless operators were also known as "The Pianists". As with their first radio sets, SOE's first ciphers were inherited from SIS. Leo Marks, SOE's chief cryptographer, was responsible for the development of better codes to replace the insecure poem codes. Eventually, SOE settled on single use ciphers, printed on silk. Unlike paper, which would be given away by rustling, silk would not be detected by a casual search if it was concealed in the lining of clothing. BBC The BBC also played its part in communications with agents or groups in the field. During the war, it broadcast to almost all Axis-occupied countries, and was avidly listened to, even at risk of arrest. The BBC included various "personal messages" in its broadcasts, which could include lines of poetry or apparently nonsensical items. They could be used to announce the safe arrival of an agent or message in London for example, or could be instructions to carry out operations on a given date. These were used for example to mobilise the resistance groups in the hours before Operation Overlord. Other methods In the field, agents could sometimes make use of the postal services, though these were slow, not always reliable and letters were almost certain to be opened and read by the Axis security services. In training, agents were taught to use a variety of easily available substances to make invisible ink, though most of these could be detected by a cursory examination, or to hide coded messages in apparently innocent letters. The telephone services were even more certain to be intercepted and listened to by the enemy, and could be used only with great care. The most secure method of communication in the field was by courier. In the earlier part of the war, most women sent as agents in the field were employed as couriers, on the assumption that they would be less likely to be suspected of