There SHOULD be infinite resistance between the wire and the can. The can goes to ground, the wire goes to the points. Zero resistance would be like the points never open. A capacitor blocks direct current, but passes alternating current. In other words, when the switch is closed putting voltage on the capacitor. current will flow until the the voltage in the capacitor equals the applied voltage. An alternating voltage will see the voltage in the capacitor rock back and forth – positive to negative to positive to negative, etc. The spark you see is a very high frequency alternating current.
A good capacitor (not leaking) will hold a charge. Back in the day when capacitors were called "condensers" and all we had were analog meters, we tested them on the resistance scale. Touching the probes to the capacitor resulted in the needle jumping and then returning to infinity. This indicates the capacitor has changed to the voltage in the meters battery (usually nine volts) and is not leaking. Leakage would show up as some resistance less than infinity. Changing to the low voltage scale the needle should jump again showing the capacitor had retained some voltage until discharging through the meter. . . 😉
