Mark

The ULB battery is a water activated device. It uses water as a conductor between the outer band and center contact to complete the circuit.
  
     
  
To activate for test, we use a small piece of wire mesh between the two contact points.

We've been installing larger ULB's attached to aircraft frame next to the recorders. The battery is larger and the beacon "pings" at a ≈1/3 slower rate. These are being used on ETOPS (extended range aircraft). I'm sure this addition is in response to Air France 447 which crashed into the Atlantic. The recorder ULB batteries ran out of battery power before the aircraft was located.

The ELT uses a "G" switch to activate the unit. It can also be turned on manually.