the central area is the mode trace. The driver's activity is displayed in this area, and is always displayed as either drive, other work, availability or rest. Earlier tachograph heads displayed the mode as a thin line in one of four concentric tracks within the activity band. These heads are known as manual heads as the activity was manually selected using the mode switch. Automatic heads succeeded manual heads, and differ from them in two main areas. Firstly, the automatic head will always display the drive mode when the vehicle is in motion, regardless of the setting of the mode switch. For this reason, the drive mode is no longer available to be selected by the mode switch. Secondly, the activity is displayed on the chart as a sequence of block traces of differing thickness. The rest mode appears as a thin line, availability as a slightly thicker line, other work as slightly thicker again and the drive trace being the thickest. The trace closest to the outer edge is the speed trace. The disc is preprinted with a speed scale and the stylus produces a mark corresponding with the speed of the vehicle at any given time. It is important that the maximum speed (Vmax) specification of the chart matches that of the tachograph head for the speed to be correctly recorded. It can be expected that a high speed trace will correlate with a tightly spaced zig-zag pattern within the distance trace. The disc is preprinted with a 24-hour scale that completes the outer circumference. The rear face of the chart is printed with a grid that enables the driver to make handwritten additions or amendments to the information on the front. Its use in accident investigation Apart from enforcing regulations, tachographs are often used in Germany to investigate and punish speeding. This practice was approved by the German high regional court in the 1990s. Also, after an accident, the discs are often examined with a microscope to discover the events that took place at a collision site. Tampering Tachographs can be tampered with in various ways, such as slightly twisting the marker, blocking the path of the arm with a piece of rubber or foam, short-circuiting the unit for short periods, intentionally preventing the detection of gear movement with a magnet, or interrupting the (older analogue) tachograph's power supply with a blown fuse to stop operation completely