Does anyone rember how to calculate impedance?

In the old days when amps and receivers had only one set of speaker outputs, if you wanted to to run more than one pair of speakers you either had to run them in parralell or in series. This varied the amplifierload. Say if I were connecting two eight ohm speaker in paralell, what load would the amplifier see? If I connected them in sereis what load would the amplifier see? The person with the correct answer will receive a vintage slide rule.
For speakers in parallel it's R1 X R2 divided by R1 + R2, where R is the speaker impedence. So two 8 ohm speakers would be 8X8=64 divided by 8+8=16 for an answer of 4 ohms. In series it's additive so two 8 ohm speakers would be 16 ohms.
Parallel Load = (Load A x Load B)/(Load A + Load B)
Series Load = Load A + Load B
You guys are genuises! So in order to avoid potential damage to the amp, it is better to connect them in sereis? Lower impedance is more likely to cause the amp to oscillate.
P.S. I was kidding about the slide rule. However, if you insist, Where can I find one?
Don't worry about the slide rule, I think I still have one. It should be packed away with the eight track and the Elcassette.
You will have load-sharing problems if you connect them in series because not all 8-ohm speakers have the same impedance curves. I would not recommend it. If you amp is tough, hook them up in parallel. Arthur
Here's the easy way to calculate resistances in parallel--use this site. Lots easier than having to use one's brain.

Speaker systems do NOT sound good when connected in series. I believe it has to do with all the signal driving the 2nd system having already run thru the 1st system's crossover. (However, it's plenty OK to connect drivers in series.) Connect the systems in parallel.

Lower impedances do not tend to cause amplifiers to oscillate, but one does need to be careful about the amp running out of current. That will happen only if you're trying to drive a quite-lo-impedance load quite loudly.
Jeffreybehr-I was giving advice to another Audiogoner. He is runnig a satellite system with two mini-monitors and two subs. It looks like there is no crossover for the subs. He essentially is running them in parrallel. He twisted the wires together and connected them to his reciever. I suggested he try running thme in series. I wanted to make sure he did not improperly load his amp. He says the speakers are rated at six ohms and his amp has selectable impedance. Sounds Like he is okay.
I learned this stuff years ago and forgot. It's not really a problem with the level of components I use. Thanks!
El just spilled the beans on your age.

Did the el cassette stay on the market for more than 6 months?

For multiple resistors in parallel:

1 1 1 1
- = - + - + - ...........
Rt R1 R2 R3

therefore four 8 ohm speakers in parallel equal 2 ohms.

I love answering electrical questions BUT remember, sound has NOTHING to do with spec's!!!!!!!!!!!!! Just listen.
Imin2u. Afraid I have to disagree with you. The ultimate test is in the listening, spec's do matter. I want a product that is technicaly correct. I have no patience with "flat earthers". If the designer is departing from the laws of physics I want to know why.
This would make it good thread of it's own.
Sorry for the igorance but what's the differnece of having the speakers hooked up in series or in parallel ?

Your question is rather open-ended as you simply say that you are putting speakers in parallel & speakers in series. You do not mention anything about the type of speaker being used.
As you might remember, speaker impedance is freq. dependent. What are called "nominal 8 ohms" speakers are 8 Ohms over a very small range.
Further, what are the resp. efficiencies of 2 speakers i.e. what is their dB SPL measured @ 1m for 1W input? Depending on this, the 2 speakers demands on the amplifier could be very different. The one that is more inefficient could suck up all the juice from the amp leaving the more efficient one clamoring for power. This'll probably show up as unequal SPL of music for a volume knob setting.
The speaker x-overs will play a dramatic role in how they load or do not load the amplifier output impedance. This will make the effective speaker load difficult to calculate @ the amp end.
3rdly, your speaker wire will also figure into this equation. Using a cable that is highly capacitative could send your amp into oscillations (unless the manuf. ensures stability under highly capacitative loads). Using a highly resistive cable will kill your amp's damping factor & wreck the bass. This'll show up as flubby bass & as well as inadequate bass. In adequate bass will cause you to crank up the volume *possibly* furthering amplifier thermal runaway.
I would suggest that you use 2 speakers are identical in efficiency, variation of load impedance & type of x-over used, if possible. If not possible then try to use speakers are nearly identical so that the amp can service both speakers just as well. If neither is possible then ensure that you have a honker of an amp that can put out gobs of current to drive virtually any speaker & be stable at the same time!
Foxtrot-Parallel is with both speaker wires connected to the amps speaker terminals. Series means one speaker lead to one speaker, Then you run anther wire off that speaker to the other speaker.
Bombaywalla-You guys never cease to amaze me with the depth of your knowledge. See the thread on "what makes my reciever hum." I gave this guy some advice and wanted to be sure I did not make his receiver blow-up.

He was essentially trying to run an outboard woofer without the benifit of an outboard crossover/ amp. I would purchase a multizone amp if I were trying to run mutliple speakers. They also have receivers that allow you to run two pair of speakers at the same time.
So Gregadd, you're telling me that you either 1) don't trust your hearing, or 2) worry about what you can't hear.
I trust only my ears! Evaluating components is an art. Designing and building them is a science.
Since no component system or room is perfect once I've identified a problem, how do I correct it? Do I bring home every product and insert it in my sysytem? Suppose I want more bass. Do I get better speakers or do I make room improvements? Measurements can't take you all the way home, but they can eliminate the pretenders.
It can also save you money. Many products use expensive overkill to solve problems. Big is good then bigger must better. A few measurements could tell you at a certain point no signifcant improvement would occur. It's no doubt in my mind that many cables are using exotic materials when it would better to install better RFI Shielding and better connectors. Most cable companies don't even supply basic measuremnts like capictance or the cables ability to reject radio waves. Maybe they should do some measuring before they charge us megabucks for upgrading to platinum.

One of my fun things to do is look at systems on audiogon. I can't belive the horrible speaker placement. Speakers so far apart they couldn't possibly be getting a proper stereo image. Cables that cost more than thier components, dipole speakers pushed up against the wall. Almost every Martin Logan I saw was too close to the wall. ( I only hope some of these were posed for sake of the picture).

We have to pay some attention to the measurements.Let's not even mention how some audiophiles idea of "good sound" is ludicrous.