HP 640 ProBook G3 Series Maintenance And Service Manual page 22

Component
(3)
(4)
(5)
14 Chapter 2 External component identification
Wireless button
Volume mute button
Fingerprint reader (select products only)
Description
Turns the wireless feature on or off but does not establish a
wireless connection.
A wireless network must be set up before a wireless connection is
possible.
Mutes and restores speaker sound.
Allows a fingerprint logon to Windows®, instead of a password
logon.