In order to monitor cathodic protection, we must be able to contact the metal of the subject pipeline or structure.

There was a time when contact was achieved by driving a steel rod into the ground from above the pipeline, and making temporary contact by piercing the coating. The main reason for discontinuing this practice was the physical damage that was possible to the pipe metal itself, and that damage that was caused to the coating.
Most pipelines now have provision for contact through electrical conductors connected to the subject metal in a variety of ways.
The most common is a process known as cadwelding, which can result in a low resistance, permanent, electrical bond between the copper conductor and the steel of a pipeline.
The disadvantage of this type of connection is that it needs considerable skill to achieve a good connection, in some field conditions.  The joint must be carefully inspected and the integrity of the coating must be tested before backfilling.
We are all aware that copper and brass are more noble than steel which will tend to disolve if coupled together in an electrolyte. Cadwelding introduces such a ' bi-metalic coupling' to the surface of the pipe and care must be taken that all metal is separated from the electrolyte by a chemically impervious, electrically resistant coating.  This coating must be compatible tho the pipeline coating and to the insulation on the copper conductor cable.

The use of copper conductor cables also introduces the possibility of a bi metalic coupling if its insulation is not perfect.  It should be remembered that the voltage that we are measuring is between the potential of copper in a saturated solution of its own salts, and steel in the local environmental electrolyte.
If the conductor to the pipe is severed, then the voltage that we measure will be that between copper in a saturated solutionof its own salts and copper in the salts that are present in the local environment. This voltage will be very low, as the difference between the potentials, will be small.
Readings can be very confusing however as they are sometimes affected by the cathodic protection current.  Charges will be passing onto the broken copper tail which is still attached to the steel of the pipe, due to the galvanic activity, and this will cause a variation in the potential of the ground in the immediate vicinity. The extent of this area of influence depends on the area of contact between the copper and the electrolyte, and the resistance of that electrolyte.

If the conductor is not completely severed, it will definitely draw currentfrom the ground, and this will have a significant effect on the measured voltage if the insulation damage is close to the electrode position.

The contact between the conductor and the subject metal must have a low electrical resistance, as it may be used for measuring current.

The best test facility is direct contact with the pipeline at a riser but there are many sections of pipeline which are buried with no riser.

There was a period when some test posts were connected to two conductors which contacted the pipeat two locations exactly 100m apart.  The purpose was to enable current direction readings to be taken, but although I saw several attempts to do this, I never saw it done successfully, or was never able to obtain meaningful readings myself.  I read and understand the theory behind this type of measurement, but the application seems impractical in field work.

The electrical resistance of the pipeline itself is extremely low, for example a 4" dia. steel pipeline is 0.141 ohms per mile and a 24" dia. pipeline is an incredibly low 0.0161 ohms per mile.(Peabodies)  If we are dealing with other structures such as storage tanks we can never consider the resistance of the metal itself, as a significant feature in cathodic protection calculations.

It therefore follows that the POTENTIAL of the pipe metal does not vary significantly, over a 2km section of continuous welded steel pipeline, with the diameters quoted.  This matter was debated during the application of over-the-pipeline potential surveys, conducted in the UK, on high pressure, welded steel, gas mains.

I was part of a team that carried out the field test which resolved this matter, on a 2km section of 24" dia. welded steel, coal tar enamel coated, buried pipeline.  This pipeline was protected by impressed current cathodic protection which was switched on continuously during these tests.

The negative pole of a high resistance voltmeter was connected to the test post conductor terminal at the top of a test post at location A.
A standard copper/copper-sulphate electrode was placed in a fixed position at location A.
A reel of armature wire was used to connect the electrode to the positive pole of the high resistance voltmeter and the reading was noted.
The wire was reeled off the spool and used to make contact with a standard copper/copper-sulphate electrode at location B, which was 2km distance from location A.
A changed voltage was noted on the meter, which was still connected to the test post at location A.
The change of standard electrode positions had significantly altered the recorded voltage.

The armature wire was then used to connect the negative pole of the voltmeter to the distant test post at location B.
The positive pole of the voltmeter was then reconnected to the electrode at location A.
The voltage on the meter was identical to the first reading.
Altering the position of contact to the pipeline, by a distance of 2 km, had no detectable influence on the voltage measured.
The meter was taken to location B and connected between that test post terminal and electrode B.
The voltage recorded was identical to the second voltage of the test, confirming that the location of the electrode is the only significant feature.
The positive pole of the meter, at location B, was then connected to the armature wire which was connected to the electrode in the fixed position at location A.
The voltage recorded was identical to the first voltage recorded confirming, once more that the point of contact to the pipeline has no detectable effect on the recorded voltages.

The discussions culminating in this test, resulted in a re-appraisal of test post locating within the operating company.  It was decided that fewer test posts were needed, and that the priority importance was access to the test post locations.

The best form of test post, for a steel pipeline consists of a steel bar welded directly onto the pipeline metal and protruding through the surface of the ground directly above. This is protected by encasement in a concrete block, which includes a vertical 4" dia.pipe filled with the local ground material.  The standard electrode would always be placed in the top of 4" pipe for the purposes of periodic voltage measurements.