Ground Lead Length; Figure 2- 1: Waveform Distortion From Ground Lead Length - Tektronix P6245 Manuel D'utilisation

Operating Basics

Ground Lead Length

When you are probing a circuit, you should always use as short a
ground lead as possible between the probe head and circuit ground.
The series inductance added by the probe tip and ground lead can
result in a resonant circuit; this circuit may cause parasitic "ringing"
within the bandwidth of your oscilloscope. Refer to Figure 2- - 1.
Low-inductance
Ground

Figure 2- 1: Waveform Distortion from Ground Lead Length

Ground Lead Inductance
When you touch your probe tip to a circuit element, you are
introducing a new resistance, capacitance, and inductance into the
circuit. Refer to Figure 2- - 2.
2- 2
Three-inch
Ground
Six-inch
Ground
P6245 Instruction Manual