mnem
Arrgh,
Those old capacators were marked with the outer CAN end, but NOT because of noise pickup. It was so you could avoid having high voltage on the can (which often had fairly poor insulation) where it could be a shock / leakage (burning) hazard.
At signal and relevant noise frequencies both ends of the capacitor are a the same potential bue to the low impedance of the capacitor.
There
was no "can" on old style paper capacitors of the type Carlson shows in his video.
A sandwich of foil & waxed paper was rolled up, & the lead connection to the foil that appeared on the outside was the one marked on those capacitors.
The later versions had metal deposited on the paper, & were commonly termed "metallised paper" types----these were mostly encased in some style of plastic, but a fairly large
minority did have cans, & a rubber "bung" on one end!
The latter weren't that marvellous----in the late '50s/early '60s, Marconi were inordinately fond of them, & I remember changing a lot which had prematurely failed, mostly with the paper capacitors which they had supposedly supplanted.
I remember reading the same thing "Mr Carlson" was talking about, in a "hand me down" 1948 ARRL Handbook, so it's not "just something he made up".
Another point made in a later version of that tome, was that bypass capacitors were often "awkwardly" mounted across Octal, 6pin, & (large) 7 pin valve sockets, to use the earthed outer layer as a shield between grid & anode circuitry.
Rewiring to a "neater" arrangement would often cause instability in such circuits.
I take a lot of Carlson's stuff "with a grain of salt" but I think he has it right, at least as far as the old caps are concerned---modern ones, maybe not so much!
The low impedance of the capacitor seems intuitive, but capacitors of that construction have a fairly substantial inductive component, & can be relatively high impedance at higher frequencies.
In 1973/74, when we were modifying some 1959 vintage Marconi TV transmitters for PAL colour, we decided to replace a couple of 0.47uF coupling capacitors in the two Submodulators.
In both units, the originals (they were the 'metal can rubber bung' type mentioned earlier), which, though they tested OK, were in a parlous physical condition, with the bung almost falling out.
To this end, we ordered some new caps.
What we received were two new 0.47uF "Styroseal" Polystyrene caps, & lovely things they were, with a clear plastic body, where you could see the internal construction.
They also had a black line, just as the old ones did to show "which way round" to fit them.
With high hopes, we fitted them, then did a swept frequency response across the video spectrum.
At around 2MHz, the thing looked like a notch filter!!!
Hurriedly restoring the horrible original cap, that Tx was restored to service!
We re-ordered, specifying "No Styroseal", & got "bog standard" old waxed paper caps, which worked perfectly!
We must have done a good job with the mods, as the old dinosaur hung in there till the early'90s!