Many US kitchens today have an instant hot tap, which appears to be the same concept as this.

You have your normal hot and cold water faucet, and there is a second faucet, with a deliberately difficult to activate on-knob, that dispenses near boiling water for rinsing grease off dishes or making coffee/tea.

Same concept, just the process is under the sink somewhere, rather than visible, and we don't call them Zips.

At least in this view.

in New Zealand, in community halls, schools, hospitals, office kitchens and the like. Mostly plumbed-in modern versions, though models like that shown can still be found.

The woman is filling the Zip from the cold tap - the other tap is the hot water tap. (Water for household use, including the bathroom, is heated from a separate, large, hot water cylinder.)

The Zip is for boiling water only - you can see in the photo the tap for releasing it, and the outlet, at the bottom of the heater. When at a community meeting, or dance etc, someone had to remember to fill and turn on the Zip well before supper, because it took a while to reach temperature (and the whistle to blow). It wouldn't do to make people wait around for a hot cuppa!

Atlas is a well-known New Zealand brand of stove - we have a modern Atlas. I don't know about elsewhere, but stoves quite often have an electrical outlet built in, for perhaps boiling an electric jug. It doesn't effect stove usage at the same time.

New Zealander here to explain---Tcrosse is correct, the whistle at the top is to let you know the water has boiled, later models switch off themselves. Bill T is also correct to say it is a safety pressure release system.These Zip water heaters are a New Zealand icon and still in use today in many public rooms and club houses, as it is a way to heat a small amount of water quickly

I can't speak to 1930s New Zealand specifically, but in many countries, it is common practice to have one tankless heater in the kitchen and another in the bathroom, with the latter known as an "electric shower". And plenty of old homes in the US have the water heater next to the stove, as a fossil remnant of a sidearm installation.

This water heater appears to be a storage tank, not only by its cylindrical shape, but from the thinness of the electrical cord (tankless heaters require a more powerful element, and therefore a bigger cable). The fact that it has a sight glass makes me think it is non-automatic; modern pressurized water tanks ordinarily contain no air bubbles, making this feature redundant.

It's interesting that the power cord is plugged into an outlet on the stove, rather than on the wall. I assume this means the water heater is controlled by an energy management relay, which prevents the water heater and oven elements from being on at the same time.

Did they have one for the bathroom too or was that just a kitchen heater? Love that old stove!

No granite countertops. No glass-mosaic backsplash. No stainless steel appliances. No terrazzo floors. Apparently, no open concept either. However did one survive in that benighted long-ago?

Australia based Zip Industries is still in business making a variety of compact "Boiling, Chilling, and Sparkling Water" devices in much the same genre as the wall mounted device here.

I will guess the whistle is part of a safety pressure release system. Always nice to know when your water heater is about to blow up.

Classic 1930's fingerwave hairdo worn by our pretty miss. Looks like it's tea time.

That looks like a whistle at the top of the gauge glass on the water heater, possibly to let you know when steam was up.