Almost private: Pen Registers, Packet Sniffers, and
Privacy at the Margin
The word pen record originally referred to a device for recording telegraph signals on a strip of paper. Samuel F B Morse's 1840 telegraph patent described such a register as consisting of level holding an armature on one end, opposite an electromagnet with a fountin pen, pensil or other marking tool on the other end, and a clockwork mechanism to advance a paper recording tape under the marker.
The term cable register came to be a generic term for such a recording machine in the later 19th century. See for example, Frank Wood's Telegraph Register. Where the record was made in ink with a pen, the term pen register emerged. By the end of the 19th century, pen registers were widely used to record pulsed electrical signals in many contexts. For example, one fire-alarm scheme used a "double pen-register", and another used a "single or multiple pen register".
As pulse dialing came into use for telephone exchange, pen registers had obvious applications as diagnostic instruments for recording sequences of phone dial pulses. With the passage of time, any tool that could be used for this purpose came to be defined as a pen register.