Does electricity go through non-metal materials or not?

Question

Ever since I was a little boy, I've always had this idea that electricity can only go through metal materials. For example, if I put a 100% metal screwdriver into the socket in the wall, I'm going to get electrocuted. Logical. Or if I take a long wire of copper (or even gold) and hold it with my hands and then stick the far end into a powered-on toaster, I'm going to get zapped.

Also ever since I was a child, I've had this screwdriver which (of course) has a metal "main part" (for the actual screwing), but whose "hand part" is made of what looks like glass or plastic. It's transparent. And there's some kind of little "spring thingie" inside it near the top. This has always confused me. I think at some point, an adult told me that it's a special screwdriver which is safe to use when there's live power. But is it?

Why is that "spring thingie" necessary at all? Isn't simply the glass/plastic cover around the metal enough to insulate my hands from the electricity that only travels through metal? I don't understand why there would be a need for some "special" kind of screwdriver with a spring inside it to protect me from electricity when it doesn't travel through non-metal materials anyway.

And even if this is such a special screwdriver, how can I know that it actually still works after all these years? How could I trust it even if it were brand new? What is it about that spring that somehow makes it safe to use it on live power?

I vaguely remember that classic story about the famous scientist who went out with a kite in a thunderstorm and got zapped by lightning hitting the kite, then traveling through the non-metal string to his hands. But did that really happen? Or is that a major misunderstanding on my part? Or did he somehow use some kind of metal wire (unlikely)? This story seems to go completely against my understanding of electricity, in that it apparently does travel through non-metal materials.

Answer 1

Your question has several sub-questions. I'll try to address them to the best of my knowledge (there are other people more knowledgeable than me on the subject).

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Spring Thingie

Regarding the spring thingie, I think you are referring to this:

Figure 1: mains Tester screwdriver (source: RS website)

The idea is that you can use this to test if a socket has a live wire in it (or which is the live wire). If you press this on a live wire, the idea is that the lamp (usually a neon) will illuminate and you will know that there is voltage difference.

By itself, the spring (or in general this type of screwdriver) is not safer than the other types of screwdrivers with a solid insulator handle. More specifically, the mains tester screwdrivers, will be able to conduct electricity at lower voltages, compared to the other type (see below )

Figure 2: electrician screwdriver with solid insulator handle (source: amazon.com)

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Regarding electricity travelling through non plastic materials.

First of all, there can be voltage difference between any material. However, each material has a different resistance to allowing electrons travelling through it.

That property is Dielectric strength and its measured in $\dfrac{V}{m}$. The higher the value of Dielectric strength, the higher is the required voltage across two surfaces to allow electrons to travel (essentially create an arc). So for conductors the value is really small, while for insulators it increases. So (at least to my knowledge, I am not an expert like Transistor):

current can travel through any material, if the voltage differential is high enough.

So coming back to the example of Benjamin Franklin with the thunderstorm, a typical lightning flash is in the order of about a few hundred million Volts. This is a very high voltage differential, and as a result, you'd either require a thin but very good insulator, or a thicker but not as good insulator.