I've never found it to be necessary. But it happens to be the law.
7 seconds sound clip from the The Tingler (1959) movie soundboard.
You can hear this line at 00:04:15 in the Blu-ray version of the movie.
Quote context
[...]
- Does it always kill them in the chair?
- I've never heard of it failing.
- Well, in the chair, does it hurt them?
- Not if it's done properly. At least, I don't think so.
- Well, if it works, then why this?
- I've never found it to be necessary. But it happens to be the law.
- An autopsy must be performed immediately after the execution.
- Interesting. This man's vertebrae are cracked. They're nearly splintered in two.
- Electricity had nothing to do with this.
- I've seen this phenomenon many times in people who were badly frightened just before they died.
[...]