Fuses fuses fuses

Of course engineers have measured fuses.

Modjeski tested the resistance of the Hi-Fi tuning fuses in both directions, and found a difference of 0.038 milliohms. I don't know about 5%, but 0.038 milliohms is an EXTREMELY small difference. I have no doubt that fuse directionality proponents will say the difference in sound is due to other factors. Like what? Oh yeah, wire directionality ;-).

Roger called and talked to both the Hi-Fi Tuning fuse U.S. distributor, and the fuses German designer. Neither knew what the term "high breaking capacity" means. Roger strongly advises against putting a Hi-Fi Tuning Fuse in a DC circuit (tubes run on DC), as a fuse lacking hbc will not protect an amp---by blowing, when a tube goes bad---as they all eventually do. Even a new tube with a short will blow a fuse capable of performing that function, which the HFTF in a DC circuit is not.

Roger Modjeski characterized  the Hi-Fi Tuning Fuse as "junk". But then he's delusional ;-).

Geoffkait 7-16-2017 (quoting the HiFi Tuning paper)
There is a measurable difference in directivity of fuses. Mostly that will be due to the way the melting wire is manufactured. The difference is in the range of 5 % . That is in the range of variations due to the factoring process, but the difference is measurable with all types of fuses.

Almarg
In fact, all or nearly all of the directional differences in resistance were vastly smaller than 5%, with the exception of the "standard glass fuse." However, even in that case, if per my comment and Roger's comments the OVERALL resistance for both directions is insignificant, the DIFFERENCE in resistance between the two orientations will certainly be insignificant.

Roger Modjeski
From the Tuning Fuse data sheet their 2 amp slow blow 5x20 fuse has a resistance of 24.077 milliohms in one direction and 24.115 in the other direction and 26.257 in the holder. If a butterfly flew by while the measurements were taking place we might see a bigger difference than the 0.038 milliohm difference in direction. Of course it might depend on which direction the butterfly was flying. But never mind, the direction measurements were made with DC and we are using these fuses in AC circuits. Perhaps if the butterfly flies clockwise vs counterclockwise there will be a difference.

Sorry I just had to put that in to keep up with all the humor that has been presented here.

[Note: 0.038 milliohms is 0.000038 ohms]

Almarg
Regarding the measurements described in the HFT paper ... which purport to support the notion of fuse directionality:

IMO those numbers are so miniscule as to be:

(a)Laughable.

(b)Very possibly attributable to changes in the voltage of the battery in the measurement meter, from measurement to measurement (each measurement imposing a slight drain on the battery), and from minute to minute. Or if the meter was AC powered, to the very slight differences in AC line voltage that may occur from minute to minute, as various loads are turned on and off at nearby locations.

(c)Very possibly attributable to differences in contact pressure and contact area between the meter’s probe tips and the contacts on the fuse. The paper presents separate measurements of fuse resistance as measured in a fuseholder (for just one direction), indicating that the direction-related measurements were performed by touching the meter leads directly to the contacts on the fuse.

(d)Perhaps even contributed to by differences in the resistance of the measurer’s body, that would have been paralleled with the resistance of the fuse if he or she had fingers on the probe tips and/or the fuse contacts while the measurements were being taken.

(e)If Geoff’s comments about all wires being significantly directional are to be believed, then these differences would be totally swamped by both the resistances and the alleged direction-related resistance differences of the vastly longer associated wiring. In the case of mains fuses, that would include the power transformer and the power wiring in the component, as well as the power cord and the AC wiring in and outside of the house....

.... Yet despite all that, and despite similar comments about the HiFi Tuning measurements that have been provided in numerous other posts going all the way back to the "Fuses that Matter" thread that began in 2012, Geoff continues to assert that the HiFi Tuning measurements support the notion that fuses are inherently directional.