A probe can be any conductor used to establish a connection between the circuit under test and the measuring instrument. This conductor could be a piece of bare wire, a multimeter lead or a piece of unterminated coaxial cable. These simple probes are not good probes because they neither extract minimal energy from the circuit under test nor transfer it to a measuring instrument with maximum fidelity.
The bare wire can load the input amplifier with its high capacitance and inductance or even cause a short circuit. Multimeter leads are unshielded and are often susceptible to stray pickup. Unterminated coax will severely capacitively load the circuit under test and is usually resonant at certain frequencies and does not allow faithful transfer of the signal due to reflections.
John Kobbe dealt with this issue and writes about his probe invention on his My Early Tektronix Days story page.
"Another anticipated problem with passive probe, we knew a signal bounced back and forth so we could not use the standard probe. While thinking about things to do like some damping resisters. Why not us a fine resistance wire as the center conductor. This time we really lucked out, it worked better than anticipated. It was even patent-able as was unblanking and sweep circuit."
The 10th Tektronix patent was 2,883,619 Electrical Probe by John R. Kobbe and William J. Polits awareded on April 21, 1959 for use of a resistive center conductor. Click on the image to view the PDF.
Tektronix makes a great number of different analog and digital probes. These include general purpose passive voltage probes, active voltage probes, high voltage passive probes, 50 ohm divider passive voltage probes, differential probes, passive current probes, active current probes, and digital probes. This probe chart shows a variety of different probe types.
Tektronix offers a number of tutorials on probes and issued this Measurement Concepts book on Probe Measurements in October 1969. It is available as parts of our All 26 Concept Books CD on our ebay store.
This current probe specification chart was made in 1961 by J.A.M. If you know who J.A.M. is please let the museum know. Click on the image to view the PDF.
One probe that has been featured in a movie is the Tektronix P600X (P6006, P6007, P6008, and P6009). This probe is used to check out the AE-35 unit in the 1968 film A Space Odyssey. This clip appears at 59:26 in the movie.
This page from the 1968 Tektronix calendar featured Tektronix probes. Click on the image to view it full size.
We have a number of different probes, probe types, and different generations of probes on display at the museum.