It’s all about the analogies, Rabbi Chaim Jachter says.

Rabbi Jachter is explaining some of the basic principles that Orthodox rabbis use to decide the questions of if, when, and how electricity can be used on Shabbat.

Rabbi Jachter leads Shaarei Orah, the Sephardic Congregation of Teaneck. He also is on the faculty of the Torah Academy of Bergen County in that town. A couple of years ago, he spent an intensive post-finals week studying these questions of halacha and electricity with a select group of students. These sessions formed the basis of his new book, “The Power of Shabbos: Shabbat and Electricity in the 21st Century.” (The title uses both Shabbos and Shabbat — the Ashkenazic and Sephardic pronunciations — to be “ecumenical,” he said. “I typically write using modern Israeli pronunciation, but Shabbos has a bit more of a cozier connotation.”)

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The book starts with electric light bulbs — the way electricity first entered daily life. (The world’s first electrical power station opened in Manhattan 140 years ago, powering street lights downtown, on Pearl Street.) But it deals with 21st century questions, including voice-activated commands, Israel’s need for 24-7 public relations, and running a website.

“You have to shut down your business on Shabbos,” Rabbi Jachter said. “Does that mean only a brick-and-mortar business or even a virtual business? Even if it’s passive, you’re sitting in your dining room having cholent but your virtual business is going ka-ching ka-ching, so you’re making a mint while you’re singing zmiros?”

Rabbi Jachter said the analogy for keeping a website open on Shabbat is a trap set before Shabbat that captures an animal on Shabbat. The Mishna discussed this case 1,800 years ago. The stricter school of Shammai said it wasn’t allowed, but the school of Hillel permitted it. And halacha — Jewish law — follows the rulings of the school of Hillel.

“It’s the same thing here,” Rabbi Jachter said. “You set the bait to get the customer. It’s a good precedent.”

That said, “It’s nice to shut it down. It’s kavod Shabbos — honoring Shabbat. B & H” — the Satmar-owned Manhattan electronics store — “shuts their website for Shabbos, which is no small matter. Even on Black Friday, they shut down. It’s really something that’s not required, according to the law, at least the consensus opinion, but it’s a beautiful thing to do.”

(It’s worth noting that while in this conversation Rabbi Jachter is discussing what he sees as the halachic bottom line, each chapter in his new book traces its issue from the original talmudic sources and presents the full range of Orthodox rulings.)

Rabbi Jachter said that one of the goals of his book “is to show that halacha is doable. At the beginning of the 20th century, critics were saying that you can’t keep Shabbos in a modern society. Especially in Israel, when you’re running a country. How do you run a hospital? How do you run the police? How do you run the army? How do you run an electric power plant?”

Or how do you run a farm?

The Talmud prohibits milking cows on Shabbat. In the post-talmudic Middle Ages, it became customary for Jews to hire non-Jews to milk their cows. But what worked in a European shtetl, where Jews and gentiles were neighbors, was less practical in a kibbutz in pre-State Israel with rising intergroup tensions. For some decades, Orthodox authorities debated whether it was allowed to use milking machines, and if so, how they should be set up.

But in the 21st century, “robotics solves the problem,” Rabbi Jachter said. “The animals are now trained so when they feel the need to be milked, they walk to the milking machine and an automated milking device comes down and gets the milk. And you don’t have to do anything.”

“The beauty is that there’s a precedent for every one of these new issues. I see it as God’s hand. How in the world can you have a precedent in a book written 2,000 years ago for every new issue that comes up? To me, it’s mind boggling. It drives my fascination with contemporary halacha.”

Take a rather off-the-wall case that shows up in the talmudic tractate Sanhedrin. You tie a person up in front of a dam. You release the water. He drowns. Are you a murderer?

Did the water hit him directly or indirectly?

It only counts as murder if he was killed by the direct force of the water.

“This sounds like a crazy case, but opening up a refrigerator is very similar — you’re removing the barrier that keeps the hot air from coming in, and it makes the compressor go on earlier than it would otherwise,” Rabbi Jachter said.

But since you caused the compressor to go on indirectly — and since the goal in opening the refrigerator door wasn’t even to turn on the compressor, an additional ameliorating factor — it’s not a problem.

In the late 1970s, the Zomet Institute in Israel started building technology based on this principle of indirect action. Instead of switches that respond directly to being pressed, electronic circuitry notices, after a delay of some seconds, that a button has been pressed. Over the decades, Zomet created a parallel set of technology designed to be operated on Shabbat.

“Religious hospitals use it.” Rabbi Jachter said. “The police run on it. Doctors in Teaneck have phones with this technology.”

Other technological changes also have made some uses of electricity less problematic on Shabbat. The analogy used by rabbis considering the first electric light bulbs was lighting a fire, or heating metal until it glowed — both clearly prohibited actions. After all, the essence of the incandescent light bulb is a glowing filament.

What, though, is the essence of a fluorescent bulb, or an LED light?

Rabbi Jachter believes it’s enough like fire to be prohibited — but the prohibition is less strict. Which makes a practical difference.

“I had to take my wife to the hospital on Rosh Hashanah a couple of years ago and at a certain point I had to use the bathroom,” he said. “The bathrooms in Englewood Hospital are electrified — the light turns on automatically.”

It went on automatically — but since the LED bulb wasn’t the equivalent of fire — according to one leading Orthodox authority, though not all — Rabbi Jachter used the bathroom, even though the light went on when he entered it.

“The Power of Shabbos” is only one of four titles Rabbi Jachter has published in the past year or so. “Thank God, covid was a very productive time,” he said.

His others include “From Chaos to Kingship: An In-Depth Exploration of Megillat Rut,” also based on TABC classes; “Bridging Traditions: Demystifying Differences Between Sephardic and Ashkenazic Jews,” a reflection of his years as an Ashkenazi rabbi leading a Sephardic synagogue; and the fourth, “The Halachic Haircutting Handbook: A Breakthrough Exposure of an Obscure Mitzvah.”

“I don’t know if it’s too interesting to you,” he said, referring to this reporter’s beard, “but for those of us who actually shave it’s a fascinating topic.”

The rules about shaving go back to Leviticus 19:27: “Do not destroy the corners of your beard.” Maimonides clarified that “destroy” means with a razor, but using a scissor — that is, the action of two blades — is allowed.

That led to a debate in the 20th century between advocates of electric shavers — who said the mechanism worked like a scissor — and opponents, who said electric shavers weren’t kosher, or had to be modified before being used. Shaver supporters included Rabbi Joseph B. Soloveitchik of Yeshiva University, where Rabbi Jachter was ordained, and Rabbi Moshe Feinstein, who was seen as America’s premiere Orthodox halachic authority until his death in 1986.

“During the pandemic, my son Binyamin and I sat down and worked on this very, very hard,” Rabbi Jachter said. “And we built a connection with a top engineer at Philips Norelco, who gave us his videos and studies and PowerPoint presentations. So I’m convinced that all electric shavers are permitted. They all function as scissors.

“It was a tremendous relief to know that we’ve been doing it right all along.”