Relay in series
The minimum voltage of one relays is about half of the supply voltage, this allows to connect two relais in series. The internal resistances act as a voltage divider. Both relays are equal in design, so both operate with 4.5V. If a third relay is integrated in the circuit, the minimum voltage is no longer reached, so that the current ow is too low for operating the relay. The relays remains inactive. Our example circuit does not really make sense, as the contacts end up in nothing and even some of the contacts are connected in some way. Its only a proof of concept design, to make curious for own experiments. When the switch is closed, both relays pull and both control LEDs light up.