using multimeter ohm resistance can be read directly from the cable, but this require to access wires; instead, having a constant current load ( eg. a led lamp of about 1A ie. 5W ), resistance of the cable can be measured indirectly using a usb current meter normally these devices comes with two side on for power input with a usb A cable male and microusb female and another side with output female A to attach the load.
(usb meter)
male USB A---+--------------+
POWER SOURCE | 5.13V 1.312A |---female USB A ---- to load
female microUSB---+--------------+
suppose want to measure a male USB A to male micro USB cable resistance
- connect load on female USB A ( use eg. a lamp with constant load of about 1A )
- connect usb meter to power source using its male USB A cable ( direct connection )
- write down measured voltage and current ( eg. V1=5.13V I1=1.313A )
- disconnect usb meter from power source
- connect usb meter to power source using USBA-micro cable which resitance want to know
- write down measure voltage and current ( eg. V2=4.90V I2=1.217A )
- compute voltage drop
V = |V2 - V1| = |4.90 - 5.13| = 0.23V
- compute current mean
I = |V1 + V2| / 2 = |1.217 + 1.313| / 2 = 1.265A
- apply ohm law V=RI to find R
R = V / I = 0.23V / 1.265A = 0.181ohm
- from the example above can be seen that our load receive lower current 1.217A vs. 1.313A due to the presence of the cable. ( bad cables introduces high resistance reducing current available for the load )
- when connect a smartphone to charger using a cable and the phone reports that it can be recharged more fast with another cable you probably found a buggy cable ( bad manifacture due to tiny wires )
- resistance generally described as
R = r * L / A
where r ( resistivity ) depends on material, so resistance increase with length (L) and decrease with section area (A)
- resistivity of copper r = 1.72 x 10-8 Ωm