What are old green & yellow wires in 1917 house?

I recently completely removed the original lath & plaster ceiling (and two lower ceilings) in my kitchen. The house is in New England and was built in 1917 with knob & tube wiring. I exposed a network of thin string-wrapped copper wires. These wires were put in when the house was built. The wires are about 1.1 mm in thickness and have white string wound around them one way and colored string wrapped around them the other way. There are both yellow and green wires, and their networks roughly overlap. (There are places where I can only see yellow crossing.) Where two wires are connected and where there is a Y in the line the copper is left exposed. The wires run around (touch) joists. A simple non-contact voltage tester indicates that voltage is present in these wires. All of my googling turned up nothing about these wires. Because European grounding wires are wrapped in a green and yellow sheath, I assume that these wires are be grounding wires.
I plan on calling an electrician, but wanted to educate myself about these wires beforehand. Thanks!