If you want to find out why relays are used as current amplifiers, in your car pull a size 00 cable from the + of the battery to a 300-Amp dashboard pushbutton switch and from there another 00 cable to the thick positive connection on the starter motor and hope that particularly in winter you don't lose too much voltage over that length of cable.
Can I have some of whatever it is your smoking. relay testers certainly exist and I use them in auto motive testing regularly. i have also linked oneI do,the comments are kindling for the fire of never ending debate and I said so in post #10. If you carefully read OP's first post,he ask's for information and explination of RELAY TESTERS and didn't ask for explination of how or why relays work. There isn't nor never was such thing as RELAY TESTERS and as I stated "anyone selling one is a con artist". As I suspected,people jumped on a chance to strut and preen by delving into theory of electricity which require's a library to fully understand. Electrical lend's itself to this back and forth,on and on because there are so many specialized fields and no single person is trained or understand's more than one or two fields. A master electrician doesn't understand dipole and ham operator understand's far less than electrician about temp poles. Duplex mean's one thing to ham and something intirly different to electrician. Undouptedly a ham could add a light switch or outlet that is safe but not neccessarly to code. This apply's to the countless fields so when specialty A make's a statment related to field D,specialty C & E can refute or at least minimumize A. And the beat go's on with first one then another stepping out to show how smart they are.
Ok,back to the relay races.
A relay allows a small or compact switch to control a heavy load that needs big contacts. Think of an ignition switch closing a circuit to energize the starter solenoid. The solenoid is a form of relay too.
Absolutely.
There was a reason programmable controllers used something called ladder logic;
