Generally, the revolvers which require an empty chamber under the hammer to be carried safely are vintage single-action revolvers like the Colt Single Action Army or S&W Model 3 and replicas thereof. These types of revolvers have either no facility to prevent a blow to the hammer from igniting the primer or a "safety" notch on the hammer that is fragile and easily sheared off by a sharp blow. This category of revolvers includes unmodified "old model" Ruger SA revolvers made prior to 1973 (Ruger will upgrade these revolvers to a configuration safe to carry fully loaded free of charge).
Most modern DA revolvers incorporate either a positive hammer block or a transfer bar which makes them safe to carry fully loaded. While very different in operation, these two systems accomplish the same basic thing: they prevent the firing pin from moving far enough forward to ignite the primer unless the trigger is pulled.
A hammer block works by placing a piece of metal in front of the hammer which prevents its forward movement. When the trigger is pulled, this piece of metal is cammed out of the path of the hammer thus allowing it to move far enough forward to ignite the primer. This system can be incorporated into a revolver with either a hammer or frame mounted firing pin. The most common revolvers to use this system are S&W's made after WWII.
A transfer bar works in the opposite fashion. The hammer does not have enough material on its front face to contact the firing pin when the action is at rest. When the trigger is pulled, a piece of metal called a transfer bar is cammed into place between the hammer and firing pin thus taking up the space between them and allowing the energy of the hammer to be transferred to the firing pin and fire the cartridge. Because of its design, a transfer bar can only be used in a revolver with a frame mounted firing pin. The most common type of revolver to use this system are Ruger DA revolvers and Ruger SA revolvers made after 1973.
There are of course exceptions to the above. S&W hand ejector revolvers made prior to and during WWII had a different type of hammer block which, instead of sliding up and down in the frame hinged from the sideplate (these are sometimes referred to as "wing" type hammer blocks). While these were much better than the fragile "safety notch" of older SA revolvers, they were somewhat prone to becoming stuck in the disengaged position and more prone to breakage than the newer type. The conventional wisdom on these guns is that they should at the least be regularly inspected for function of the hammer block and some would argue that they should be carried with an empty chamber under the hammer. The other notable exception is S&W "Centennial" revolvers such as the Models 40, 42, 340, 640, 442, and 642. These revolver have no hammer block at all. This is because these revolvers' hammers are completely enclosed by the frame thus making a hammer block unnecessary as a blow to the hammer is pretty much impossible.
