Remove a light switch to an outlet - knob and tube
The best solution is to replace all old cloth insulated wiring. 

If you can't replace a section of cloth insulated wiring the next best solution is to not touch it at all, because  deteriorating insulation will deteriorate much faster every time you touch it.

If you are forced to touch a cloth wired circuit, re-insulate with either heat shrink tubing or electrical tape everything exposed while working on it, and upgrade the breaker to AFCI. Given the likely age of any cloth wiring you encounter today, all circuits that you can't rewire should be upgraded to AFCI breakers. Unfortunately, if your circuits have any of their neutrals crossed or shared AFCI won't work and using the new breakers would still require a rewiring. As another response pointed out what you are doing is safe and legal as long as the insulation is in tact, which is always the question with this type of wire. Where the wire's only contact with anything else are the glass knobs, it is safe even without insulation unless something does come in contact with both wires for any reason. 

I completely rewired my house, the original wiring was approximately 100 years old, most of the insulation was still in very good shape, but there were also places where it crumbled to dust the moment it was disturbed.